All-solution Synthesized High-performance 2D Bi2O2Se Thin-film Transistor

Kim, Jin Seok 고려대학교 KU-KIST융합대학원 2023 국내석사

Two-dimensional (2D) semiconductors have emerged as a next-generation electronic material because of their excellent electrical and mechanical properties in the atomically thin regime. These materials with a van der Waals layered structure are particularly promising for emerging electronics capable of heterogeneous integration and flexibility. For this purpose, low-temperature, and high-quality synthesis of 2D semiconductors are essential, wet chemical synthesis has remarkable advantages in low process temperature, scalability, and cost-effectiveness compared to conventional chemical vapor deposition processes. However, it remains a considerable challenge to achieve high quality and device performance. Here, we report the wet chemical synthesis via a bottom-up process of 2D Bi2O2Se semiconductors showing a high mobility characteristic. The all-solution-based processes are carried out at a low temperature below 200 ℃ producing free-standing 2D flakes with a lateral size of over 10 μm and thickness down to 8 nm. In addition, the single-crystalline Bi2O2Se channel in a back-gated field-effect transistor geometry exhibited high mobility up to 132 cm2V-1s-1 at room temperature. Notably, this solution can be assembled into a thin film for large-area device fabrication through a simple method such as liquid/air interface self-assembly. Our demonstration provides an innovative bottom-up synthesis approach to preparing high-quality semiconductors cost- and energy-effectively.

A Study on Fabrication of High Performance Field Effect Transistor Based on CVD Graphene

박슬기 포항공과대학교 일반대학원 2020 국내박사

In this dissertation, I covered a new fabrication method of high-performance FET based on CVD graphene, which minimizes causes of electrical property degradation occurred from conventional graphene transfer and patterning process Graphene is made by carbon atoms aligning on metal catalyst, so if dielectric patterns are deposited on the metal, the graphene is synthesized only on the area where the metal surface is exposed, resulting in graphene patterns being obtained without any patterning process. When selective growth using a silicon dioxide (SiO2) patterns was conducted under conditions that single-layer graphene is synthesized on blank Cu foil (BCF), the number of layers was increased. The electrical properties of graphene significantly varies with the number of layers, so it is essential to obtain a uniform single layer graphene to achieve high carrier mobility. Therefore, Experiments to demonstrate the causes for the increase of the number of graphene layers was conducted; oxygen, hydrogen, and dielectric coverage. The effect of hydrogen was tested by controlling the amount of hydrogen flow at the synthesis stage. The synthesis of graphene by chemical vapor deposition method (CVD) consists of four stages: heating, annealing, synthesis and cooling. To control other variables, the amount of gas used in the synthesis was the same except for synthesis stage. At the heating, annealing and cooling stages, only 4 sccm of H2 was used, and at the synthesis stage, the ratio of H2 to CH4 (H2 : CH4) was used 8:20, 4:20, and 4:10 in sccm. The amount of CH4 gas was also adjusted because H2 also is decomposed from CH4. As the conditions changed to 8:20, 4:20 and 4:10, in other words, as the effect of hydrogen reduced, the increase of the number of layers was also decreased. That is, it was confirmed that the contribution of H2 became greater for some reason, resulting in the increase graphene layers compared to synthesis on BCF. To check the effect of oxygen, the synthesis was conducted using copper foil with Al2O3 and Si3N4 that were frequently used in semiconductor industries in addition to SiO2. Because of the presence of oxygen in Al2O3 film, the results on Cu foil with Al2O3 were expected to be similar to those of SiO2, and because Si3N4 film does not contain oxygen, single-layer graphene was expected to be synthesized on Cu foil with Si3N4. However, the results of synthesis demonstrated that there was no effect of oxygen in the synthesis because layer increase was found in all samples, and the degree of layer increase were similar. Except for the existence of dielectric film, the only difference between the samples used for general growth and selective growth was the percentage of exposed Cu surface. The exposed area of Cu foil used for selective growth was about 5%, i.e., dielectric patterns cover 95% of whole Cu surface. Therefore, to check the effects of dielectric coverage, additional synthesis was conducted under H2 : CH4 =4:10 using Cu foil with 50% dielectric coverage. As a result, uniform single-layer graphene patterns were successfully grown on Cu foil with 50% dielectric coverage under H2 : CH4 = 4:10. Through Raman map, it was also verified that the distribution of single layer graphene was very uniform. This is direct evidence that H2 gas more reacted with the Cu surface, i.e., the amount of H2 reacting per unit Cu area where SiO2 was opened varies depending on the dielectric coverage on Cu foil even under the same gas flow. In the fabrication of damage-free graphene FETs, parylene-C is deposited directly on selectively-grown graphene/Cu foil. Cu foil is removed in the etching solution after the deposition, then the selectively-grown single-layer graphene is transferred to the parylene C substrate. Through this process, the graphene is completely free from the deterioration of characteristics arising from the conventional transfer and patterning processes. By using the same parylene-C used as a substrate as a gate dielectric layer, graphene can be less affected by phonon scattering than the high-k material, and the devices can also be fully-flexible. Curve fitting to the diffusive model confirmed that graphene FET created by the damage-free fabrication had 10,260 cm2/V·s of hole mobility and 10,010 cm2/V·s of electron mobility. These values were comparable to those of previously reported CVD graphene based FETs and was the best compared to those of fully-flexible graphene FETs. In this method, because the process is simplified compared to the traditional graphene FETs fabrication process and there are no factors that are affected by human, the deviation of the results is small. It is also expected to contribute greatly to the mass production of reliable, high-performance graphene transistors because this method has advantages of low-cost and production of large area graphene.

Metal-Templated Crystallization of Germanium for Optoelectronic Applications

Li, Yanying ProQuest Dissertations & Theses Stanford Universit 2015 해외박사(DDOD)

Crystalline germanium (Ge) structures, such as nanowires (NWs) and thin films, have been investigated intensively in recent years due to their unique properties emerging from germanium's large absorption coefficient, high carrier mobilities, lattice match with photovoltaic material GaAs, and compatibility with standard silicon-processing technology. The dynamics of electrons, photons, and phonons in crystalline Ge strongly depend on the geometrical factors of the different device structures in which it is fabricated. A better understanding of Ge growth mechanisms in different nano- and micro-structures, and their connection to fundamental optical and electronic properties is essential in order to better exploit Ge in the design of nanoscale optoelectronic devices. However, such investigations in semiconductor NWs and thin films are limited and have mainly focused on a small set of crystal growth processes. This thesis focuses on the investigations of both the synthesis and properties of two types of structures, NWs and poly-crystalline thin films. In order to achieve large-scale arrays of relatively defect-free vertically aligned NWs, it is essential to understand the spontaneous kinking during growth. In Chapter 2, two fundamental mechanisms underlying the spontaneous kinking of Ge NWs during vapor-liquid-solid (VLS) growth will be discussed. The diameter of NWs, sidewall facets of NWs, and the capillary stability of the Au-Ge catalyst droplet play important roles in spontaneous kinking. 3-D phase field model simulations by our collaborators are combined with experimental results to confirm the kinking mechanisms. The high electron and hole mobilities and high optical absorption in visible and IR wavelengths of Ge makes it a promising candidate for ultrafast optoelectronic device applications. There are few investigations of arrays of Ge NWs in this context, and most such studies have focused on a qualitative analysis of relevant phenomena. Chapter 3-5 will present a study of ultrafast optical, acoustic and electronic properties of vertically aligned Ge NW arrays through ultrafast, optical pump-probe transient absorption measurements on dense arrays of single-crystal and relatively uniform-diameter Ge NWs. Two coexisting physical phenomena governing the spectral and temporal dependence of the detected probe signal will be discussed. In Chapter 4, ultrafast dynamics of electrons and holes, especially their strong dependence on NW diameters and photoexcitation powers, are investigated. The different interaction of electrons and holes with surface states of Ge NWs will be demonstrated, thereby leading to different methods to extend electron and hole lifetime in such nanostructures. Chapter 6 will first introduce a method to produce poly-crystalline Ge (poly-Ge) thin films via low-temperature Al-induced-crystallization. Electron backscatter diffraction (EBSD), Hall mobility, and photoluminescence (PL) measurements of the poly-Ge films and poly-GaAs films epitaxially deposited on these poly-Ge templates indicate that templates with relatively large Ge crystallites (up to 124 um2) are a low-cost alternative to single-crystal Ge wafers, albeit exhibiting somewhat reduced performance. The poly-Ge templates can function well for seeding epitaxial growth of overlying Ge nanostructures or GaAs thin film absorbers for application in photodetectors or solar cells.

Studies on the electrical and optical properties of electron doped BaSnO₃ with high electrical mobility

김형준 서울대학교 대학원 2013 국내박사

Transparent conducting oxides (TCOs) and transparent oxide semiconductors (TOSs) are optically transparent in the visible light spectral region and simultaneously have electrical conductivity. Recently, using TCOs and TOSs, many efforts have been made to combine the semiconducting functionality with optical transparency to develop transparent electronic devices such as transparent displays, touch-panels, and smart windows. Even though a lot of materials have been developed for TCOs and TOSs, it is an ongoing demanding research effort to find a better material that can exhibit better physical properties such as high electrical mobility and thermal stability. A simple cubic perovskite BaSnO3 is a wide-band-gap semiconductor with a band gap of 3.1 eV. We found that (Ba,La)SnO3 has a high-level of optical transparency with a high electrical mobility of n-type carriers at room temperature. In single crystals, mobility of 200 – 300 cm2V-1s-1 is realized in a broad doping range from 1.01019 to 4.01020 cm-3, which has superior electrical mobility among the TCOs and TOSs. Moreover, the conductivity of ~104 -1cm-1 reached at the latter carrier density is comparable to the highest value previously reported in Sn-doped In2O3. In the case of thin (Ba,La)SnO3 film on SrTiO3 (001), the maximum mobility was only ~ 70 cm2V-1s-1 due to dislocations or grain boundaries, which is originated from the lattice mismatch between BaSnO3 and SrTiO3. Therefore, if we use a lattice matched substrate such as BaSnO3, high electrical mobility can be realized in films. In order to understand the physical origin of high electrical mobility in (Ba,La)SnO3, we investigated various physical properties of BaSnO3 and electron doped BaSnO3 systems including electronic structure, optical properties, and temperature-dependent electrical properties. Based on the carrier scattering theory and our investigations, we can understand that the high electrical mobility of (Ba,La)SnO3 originates from the reduced ionized impurity scattering due to the high dielectric constant of BaSnO3 and the location of La3+ dopants away from the main conduction paths (SnO6 octahedral network). We also investigated another electron doped BaSnO3 system of Ba(Sn,Sb)O3. In the case of Ba(Sn,Sb)O3 single crystals, we found that the electrical mobility at room temperature reaches as high as 79.4 cm2V-1s-1 at a carrier density of 1.021020 cm-3. The overall reduced mobility of the Ba(Sn,Sb)O3 system as compared to the (Ba,La)SnO3 system is attributed to the enhanced free carrier scattering due to neutral impurities. Based on our experimental result that half of Sb dopants do not give free carriers in Ba(Sn,Sb)O3, the origin of neutral impurities might be due to the other half Sb dopants forming the Sb4+-like state or oxygen interstitial-related Sb defects. This indicates that La doping is a more efficient way to introduce electron carriers. In addition to the electrical properties, we found that BaSnO3 has excellent thermal stability based on the high temperature resistance measurement of (Ba,La)SnO3 thin film. We measured the resistance variation of the films at high temperature (~ 530 C) for 10 hours under various gas conditions such as O2, Ar, and air. The resistance change was less than 8% in O2 and Ar. If we consider that the resistance change originated from the oxygen in- and out-diffusion, the small change of resistance might indicate that the BaSnO3 has excellent oxygen stability. We also investigated the field-effect-transistor (FET) effect with undoped BaSnO3 single crystals to demonstrate the active device operation and study the field-induced carrier transport properties. We successfully realized the field effect transistor behavior with a liquid gating technique. Even though the on/off current ratio was ~ 104 at the moment, this indicates a possibility of controlling the free carrier density by an electric field. Based on our experimental and theoretical results from the BaSnO3 system including high electrical mobility, optical transparency, and excellent thermal stability, we believe that BaSnO3 can be a good candidate material for TOSs/TCOs in transparent high-temperature, high-power functional devices. In addition to the industrial aspects, many prospective studies are expected such as on realizing FET and two-dimensional electron gas with BaSnO3, and finding quantum oscillation and the quantum hall effect. Therefore, BaSnO3 can be a prospective oxide material for future oxide electronics research.