Deposition of High-K dielectrics on 2D-semiconductors via low temperature ALD

2D materials such as graphene and TMDCs (Transition Metal Dichalcogenides) have increased interest in the research of future electronic devices due to their excellent electronic properties. These materials can be easily exfoliated to make a monolayer which enables us to study the 2D properties of the materials such as the quantum confinement effect. In case of TMDCs, intrinsic bandgaps of the materials provide possible applications in digital logic devices. TMDCs also have ideal material properties for TFETs (Tunnel Field Effect Transistors). Having no dangling bond at the materials’ surface, defects at the heterojunction interface can be minimized enabling to obtain steep TFETs with lower subthreshold swing. To realize these properties in the devices, preparation of insulating and uniform gate oxide with low EOT (Equivlaent Oxide Thickness) is required. However, due to the inert surface nature of 2D materials, various functionalization techniques have been used to initiate nucleation of the gate oxide. However, these functionalization methods inevitably induce damage to 2D materials, changing the electronic properties of 2D materials. Therefore, for successful fabrication of 2D materials electronic devices, a more facile gate oxide deposition method with low surface defects is needed. This thesis consists of three parts. In the first part(Chapter 2), deposition of Al2O3/ HfO2 bilayer nanolaminate structures on Si0.7Ge0.3(100) by thermal ALD was studied to develop high-K oxide with lower interface defects for 2D materials. It is shown that Al2O3/ HfO2 bilayer or nanolaminate structures effectively reduced the density of interface traps more than 30% at the expense of a small capacitance drop in the accumulation. In addition, 3 orders of magnitude lower leakage currents were achieved compared to pure HfO2 layer. Second part (Chapter 3) introduces mechanism of low temperature ALD of Al2O3 on graphene. This study shows that a uniform and defect free Al2O3 film can be grown on graphene by ALD at low temperature without any functionalization techniques. The Capacitance-Voltage measurements of the 50 cycles of ALD Al2O3 films growth at 50oC showed 1.17 μF/cm2 with very low leakage current of the Al2O3 was in order of 10-5 A/cm2 which is consistent with the absence of pinholes. In the last part(Chapter 4), deposition of high quality Al2O3 and HfO2/Al2O3 films on 2D materials using low temperature ALD/CVD was demonstrated. Cmax and leakage current values of 50 cycles of low temperature ALD Al2O3 on MoS2, HOPG and Si0.7Ge0.3(001) were comparable indicating uniform and pinhole free Al2O3 films across the entire surface. To obtain lower EOT, Al2O3 (7 cycles at 50 oC)/HfO2(40 cycles at 300 oC) bilayer gate oxide was prepared on 2D materials substrates. Cmax was increased by 2x compared to 50 cycles Al2O3 MOSCAPs. In addition, Pd/Ti/TiN gate was employed to scavenge the oxygen from the oxide. Cmax of ~2.7 µF/cm2 was achieved with MoS2 and HOPG without loss of leakage current density. All 2D materials MOSCAPs in this study had lower interfacial defect density (Dit) compared to the same gate stacks on Si0.7Ge0.3(001) indicating Van der Waals interactions between the oxide and the 2D material surfaces is dominant instead of chemical bondings

Deposition of High-K dielectrics on 2D-semiconductors via low temperature ALD

2D materials such as graphene and TMDCs (Transition Metal Dichalcogenides) have increased interest in the research of future electronic devices due to their excellent electronic properties. These materials can be easily exfoliated to make a monolayer which enables us to study the 2D properties of the materials such as the quantum confinement effect. In case of TMDCs, intrinsic bandgaps of the materials provide possible applications in digital logic devices. TMDCs also have ideal material properties for TFETs (Tunnel Field Effect Transistors). Having no dangling bond at the materials’ surface, defects at the heterojunction interface can be minimized enabling to obtain steep TFETs with lower subthreshold swing. To realize these properties in the devices, preparation of insulating and uniform gate oxide with low EOT (Equivlaent Oxide Thickness) is required. However, due to the inert surface nature of 2D materials, various functionalization techniques have been used to initiate nucleation of the gate oxide. However, these functionalization methods inevitably induce damage to 2D materials, changing the electronic properties of 2D materials. Therefore, for successful fabrication of 2D materials electronic devices, a more facile gate oxide deposition method with low surface defects is needed. This thesis consists of three parts. In the first part(Chapter 2), deposition of Al2O3/ HfO2 bilayer nanolaminate structures on Si0.7Ge0.3(100) by thermal ALD was studied to develop high-K oxide with lower interface defects for 2D materials. It is shown that Al2O3/ HfO2 bilayer or nanolaminate structures effectively reduced the density of interface traps more than 30% at the expense of a small capacitance drop in the accumulation. In addition, 3 orders of magnitude lower leakage currents were achieved compared to pure HfO2 layer. Second part (Chapter 3) introduces mechanism of low temperature ALD of Al2O3 on graphene. This study shows that a uniform and defect free Al2O3 film can be grown on graphene by ALD at low temperature without any functionalization techniques. The Capacitance-Voltage measurements of the 50 cycles of ALD Al2O3 films growth at 50oC showed 1.17 μF/cm2 with very low leakage current of the Al2O3 was in order of 10-5 A/cm2 which is consistent with the absence of pinholes. In the last part(Chapter 4), deposition of high quality Al2O3 and HfO2/Al2O3 films on 2D materials using low temperature ALD/CVD was demonstrated. Cmax and leakage current values of 50 cycles of low temperature ALD Al2O3 on MoS2, HOPG and Si0.7Ge0.3(001) were comparable indicating uniform and pinhole free Al2O3 films across the entire surface. To obtain lower EOT, Al2O3 (7 cycles at 50 oC)/HfO2(40 cycles at 300 oC) bilayer gate oxide was prepared on 2D materials substrates. Cmax was increased by 2x compared to 50 cycles Al2O3 MOSCAPs. In addition, Pd/Ti/TiN gate was employed to scavenge the oxygen from the oxide. Cmax of ~2.7 µF/cm2 was achieved with MoS2 and HOPG without loss of leakage current density. All 2D materials MOSCAPs in this study had lower interfacial defect density (Dit) compared to the same gate stacks on Si0.7Ge0.3(001) indicating Van der Waals interactions between the oxide and the 2D material surfaces is dominant instead of chemical bondings

Low interface trap density in scaled bilayer gate oxides on 2D materials via nanofog low temperature atomic layer deposition

Al2O3 and Al2O3/HfO2 bilayer gate stacks were directly deposited on the surface of 2D materials via low temperature ALD/CVD of Al2O3 and high temperature ALD of HfO2 without any surface functionalization. The process is self-nucleating even on inert surfaces because a chemical vapor deposition (CVD) component was intentionally produced in the Al2O3 deposition by controlling the purge time between TMA and H2O precursor pulses at 50 °C. The CVD growth component induces formation of sub-1 nm AlOx particles (nanofog) on the surface, providing uniform nucleation centers. The ALD process is consistent with the generation of sub-1 nm gas phase particles which stick to all surfaces and is thus denoted as nanofog ALD. To prove the ALD/CVD Al2O3 nucleation layer has the conformality of a self-limiting process, the nanofog was deposited on a high aspect ratio Si3N4/SiO2/Si pattern surface; conformality of >90% was observed for a sub 2 nm film consistent with a self-limiting process. MoS2 and HOPG (highly oriented pyrolytic graphite) metal oxide semiconductor capacitors (MOSCAPs) were fabricated with single layer Al2O3 ALD at 50 °C and with the bilayer Al2O3/HfO2 stacks having Cmax of ∼1.1 µF/cm2 and 2.2 µF/cm2 respectively. In addition, Pd/Ti/TiN gates were used to increase Cmax by scavenging oxygen from the oxide layer which demonstrated Cmax of ∼2.7 µF/cm2. This is the highest reported Cmax and Cmax/Leakage of any top gated 2D semiconductor MOSCAP or MOSFET. The gate oxide prepared on a MoS2 substrate results in more than an 80% reduction in Dit compared to a Si0.7Ge0.3(0 0 1) substrate. This is attributed to a Van der Waals interaction between the oxide layer and MoS2 surface instead of a covalent bonding allowing gate oxide deposition without the generation of dangling bonds. © 201

Growth Mode Transition from Monolayer by Monolayer to Bilayer by Bilayer in Molecularly Flat Titanyl Phthalocyanine Film

To avoid defects associated with inhomogeneous crystallites and uneven morphology that degrade organic device performance, the deposition of ultraflat and homogeneous crystalline organic active layers is required. The growth mode transition of organic semiconducting titanyl phthalocyanine (TiOPc) molecule from monolayer-by-monolayer to bilayer-by-bilayer can be observed on highly ordered pyrolytic graphic (HOPG), while maintaining large and molecularly flat domains. The first monolayer of TiOPc lies flat on HOPG with a ∼98% face-up orientation. However, as the thickness of the TiOPc increases to over 15 monolayers (ML), the growth mode transitions to bilayer-by-bilayer with the repeated stacking of bilayers (BL), each of which has face-to-face pairs. Density functional theory calculations reveal that the increasing of thickness induces weakening of the substrate effect on the deposited TiOPc layers, resulting in the growth mode transition to BL-by-BL. The asymmetric stacking provides the driving force to maintain nearly constant surface order during growth, allowing precise, subnanometer thickness control and large domain growth

Solution-Cast Monolayers of Cobalt Crown Ether Phthalocyanine on Highly Ordered Pyrolytic Graphite

15-Crown-5-ether-substituted cobalt­(II) phthalocyanine (CoCrPc) is an atomically thin and flat-laying, electrically insulating molecule that can solvate ions; these properties are desirable for nanoelectronic devices. A simple, solution-phase deposition method is demonstrated to produce a monolayer of CoCrPc on highly ordered pyrolytic graphite (HOPG). A uniform and continuous CoCrPc layer is obtained on freshly cleaved HOPG by solution drop casting, followed by thermal annealing under ambient pressure in Ar in the temperature range of 150–210 °C. While the quality of the monolayer is independent of annealing time, the composition of the annealing atmosphere is critical; exposure to ambient air degrades the quality of the monolayer over the time scale of minutes. Using ultrahigh vacuum scanning tunneling microscopy, a highly ordered and flat CoCrPc layer with hexagonal symmetry and average spacing of 4.09 ± 0.2 nm is observed. The band gap of the CoCrPc, measured by scanning tunneling spectroscopy, is 1.34 ± 0.07 eV. The ability to prepare uniform, ordered, and conformal monolayers of CoCrPc molecules on HOPG represents the first step toward using these materials to seed dielectric growth on 2D crystals and provide a 2D electrolyte for the electrostatic gating of semiconductors at the ultimate limit of scaling

<i>In Situ</i> Observation of Initial Stage in Dielectric Growth and Deposition of Ultrahigh Nucleation Density Dielectric on Two-Dimensional Surfaces

Several proposed beyond-CMOS devices based on two-dimensional (2D) heterostructures require the deposition of thin dielectrics between 2D layers. However, the direct deposition of dielectrics on 2D materials is challenging due to their inert surface chemistry. To deposit high-quality, thin dielectrics on 2D materials, a flat lying titanyl phthalocyanine (TiOPc) monolayer, deposited via the molecular beam epitaxy, was employed to create a seed layer for atomic layer deposition (ALD) on 2D materials, and the initial stage of growth was probed using <i>in situ</i> STM. ALD pulses of trimethyl aluminum (TMA) and H₂O resulted in the uniform deposition of AlO_<i>x</i> on the TiOPc/HOPG. The uniformity of the dielectric is consistent with DFT calculations showing multiple reaction sites are available on the TiOPc molecule for reaction with TMA. Capacitors prepared with 50 cycles of AlO_<i>x</i> on TiOPc/graphene display a capacitance greater than 1000 nF/cm², and dual-gated devices have current densities of 10^–7A/cm² with 40 cycles

Selective Chemical Response of Transition Metal Dichalcogenides and Metal Dichalcogenides in Ambient Conditions

To fabricate practical devices based on semiconducting two-dimensional (2D) materials, the source, channel, and drain materials are exposed to ambient air. However, the response of layered 2D materials to air has not been fully elucidated at the molecular level. In the present report, the effects of air exposure on transition metal dichalcogenides (TMD) and metal dichalcogenides (MD) are studied using ultrahigh-vacuum scanning tunneling microscopy (STM). The effects of a 1-day ambient air exposure on MBE-grown WSe₂, chemical vapor deposition (CVD)-grown MoS₂, and MBE SnSe₂ are compared. Both MBE-grown WSe₂ and CVD-grown MoS₂ display a selective air exposure response at the step edges, consistent with oxidation on WSe₂ and adsorption of hydrocarbon on MoS₂, while the terraces and domain/grain boundaries of both TMDs are nearly inert to ambient air. Conversely, MBE-grown SnSe₂, an MD, is not stable in ambient air. After exposure in ambient air for 1 day, the entire surface of SnSe₂ is decomposed to SnO_<i>x</i> and SeO_<i>x</i>, as seen with X-ray photoelectron spectroscopy. Since the oxidation enthalpy of all three materials is similar, the data is consistent with greater oxidation of SnSe₂ being driven by the weak bonding of SnSe₂