Ultraclean Si/Si interface formation by surface preparation and direct bonding in ultrahigh vacuum

Silicon surfaces have been cleaned and bonded in ultra-high vacuum at a pressure in the 10-10 torr range. The bonded interfaces show extremely low contamination levels as measured by secondary ion mass spectroscopy. Nevertheless, a potential barrier could be detected at the interface spreading resistance and current vs. temperature measurements. This suggests that the barrier is caused by inevitable dislocation networks due to wafer misorientation, as well as residual oxygen at the interfac

Graphene on Si(111)7x7

We demonstrate that it is possible to mechanically exfoliate graphene under ultra high vacuum conditions on the atomically well defined surface of single crystalline silicon. The flakes are several hundred nanometers in lateral size and their optical contrast is very faint in agreement with calculated data. Single layer graphene is investigated by Raman mapping. The G and 2D peaks are shifted and narrowed compared to undoped graphene. With spatially resolved Kelvin probe measurements we show that this is due to p-type doping with hole densities of n_h \simeq 6x10^{12} cm^{-2}. The in vacuo preparation technique presented here should open up new possibilities to influence the properties of graphene by introducing adsorbates in a controlled way.Comment: 8 pages, 7 figure

Graphene on Si(111)7×7

We demonstrate that it is possible to mechanically exfoliate graphene under ultrahigh vacuum conditions on the atomically well defined surface of single crystalline silicon. The flakes are several hundred nanometers in lateral size and their optical contrast is very faint, in agreement with calculated data. Single-layer graphene is investigated by Raman mapping. The graphene and 2D peaks are shifted and narrowed compared to undoped graphene. With spatially resolved Kelvin probe measurements we show that this is due to p-type doping with hole densities of nh ≃ 6 × 1012 cm−2. The in vacuo preparation technique presented here should open up new possibilities to influence the properties of graphene by introducing adsorbates in a controlled way.DFG, 130170629, SPP 1459: Graphen

Early space experiments in materials processing

A comprehensive survey of the flight experiments conducted in conjunction with the United States Materials Processing in Space Program is presented. Also included are a brief description of the conditions prevailing in an orbiting spacecraft and the research implications provided by this unique environment. What was done and what was learned are summarized in order to serve as a background for future experiments. It is assumed that the reader has some knowledge of the physical sciences but no background in spaceflight experimentation or in the materials science per se

Experiments in Interrupted Growth Molecular Beam Epitaxy Technology

From a device structure standpoint it would be advantageous to sandwich laterally defined features between layers of epitaxially grown material. In silicon this is commonly dope by growing the bottom layer, patterning the desired feature, and growing a second layer. Unfortunately, this process has not been practical in GaAs for the same reason that there is no true MOS technology in GaAs: The. GaAs surface is irreparably damaged when it is exposed to the atmosphere leading to the formation of undesirable interface states. Heterojunction FET\u27s are feasible only because high quality epilayers are grown during a single run in an ultrahigh vacuum environment. Standard growth methods allow for variation of doping and material content only in one direction, normal to the wafer surface. Varying the material in more than one dimension without the use of prohibitively exotic equipment requires removal of the wafer from the growth apparatus for lateral processing between material growths. Thus the problem that this thesis attempts to address: How to protect a GaAs surface during a lateral processing step and initiate regrowth leaving behind an electrically invisible restart interface. The potential applications of the development of a successful interrupted growth scheme for GaAs are numerous and far reaching. Specifically it would allow the fabrication of advantageous device geometries that are not possible under single material growth runs. Although this thesis deals exclusively with ion implanted interrupted growth by Molecular Beam Epitaxy, some of the concepts arid theories can be extended to other growth methods. It is both a review of previous work and a report of our attempts at Purdue to fabricate the first interrupted growth HIGFET\u27s and MISFET\u27s. Mechanisms behind the success and failure of GaAs interrupted growth are discussed and several experiments involving passivation materials and new interrupted growth schemes are propose

Substrate Effects And Dielectric Integration In 2d Electronics

The ultra-thin body of monolayer (and few-layer) two dimensional (2D) semiconducting materials such as transitional metal dichalconiges (TMDs), black phosphorous (BP) has demonstrated tremendous beneficial physical, transport, and optical properties for a wide range of applications. Because of their ultrathin bodies, the properties of 2D materials are highly sensitive to environmental effects. Particularly, the performance of 2D semiconductor electronic devices is strongly dependent on the substrate/dielectric properties, extrinsic impurities and absorbates. In this work, we systematically studied the transport properties of mechanically exfoliated few layer TMD field-effect transistors (FETs) consistently fabricated on various substrates including SiO2,Parylene –C, Al2O3, SiO2 modified by octadecyltrimethoxysilane (OTMS) self-assembled monolayer (SAMs), and hexagonal boron nitride (h-BN). We performed variable temperature transport measurements to understand the effects of various scattering mechanisms such as remote surface phonon scattering, coulomb scattering, surface roughness scattering on the mobility of these devices. To reveal the intrinsic channel properties, we also investigated TMD devices encapsulated by h-BN. To further optimize the dielectric interface and electrostatic control of the TMD channels, we developed a novel thermal-oxidation method to turn few-layer 2D metals into ultrathin and atomically flat high –κ dielectrics. In order to optimize the performance of TMD electronic devices, it is also critical to fabricate low resistance ohmic contacts required for effectively injecting charge carriers into the TMD channel. Along this direction, we developed a new contact strategy to minimize the contact resistance for a variety of TMDs by van der Waals assembly of doped TMDs as contacts and undoped TMDs as channel materials. The developed unique method for low-resistance ohmic contacts achieved using the 2D/2D contact strategy and novel technique for high-k dielectric integration is expected to open the path to explore the rich quantum physics in TMDs 2DEGs and 2DHGs

Doped And Chemically Transformed Transition Metal Dichalcogenides (tmdcs) For Two-Dimensional (2d) Electronics

Transition metal dichalcogenides (TMDCs) as the semiconductor counterparts of gra-phene have emerged as promising channel materials for flexible electronic and optoelectronic devices. The 2D layer structure of TMDCs enables the ultimate scaling of TMDC-based devices down to atomic thickness. Furthermore, the absence of dangling bonds in these materials helps to form high quality heterostructures with ultra-clean interfaces. The main objective of this work is to develop novel approaches to fabricating TMDC-based 2D electronic devices such as diodes and transistors. In the first part, we have fabricated 2D p-n junction diodes through van der Waals assembly of heavily p-doped MoS2 (WSe2) and lightly n-doped MoS2 to form vertical homo-(hetero-) junctions, which allows to continuously tune the electron concentration on the n-side for a wide range. In sharp contrast to conventional p-n junction diodes, we have observed nearly exponential dependence of the reverse-current on gate-voltage in our 2D p-n junction devices, which can be attributed to band-to-band tunneling through a gate-tunable tunneling barrier. In the second part, we developed a new strategy to engineer high-κ dielectrics by con-verting atomically thin metallic 2D TMDCs into high-κ dielectrics because it remains a signifi-cant challenge to deposit uniform high-κ dielectric thin films on TMDCs with ALD due to the lack of dangling bonds on the surfaces of TMDCs. In our study, we converted mechanically ex-foliated atomically thin layers of a 2D metal, TaS2 (HfSe2) into a high-κ dielectric, Ta2O5 (HfO2) by thermal oxidation. X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and atomic force microscopy (AFM) were used to understand the phase conversion process. Capacitance-voltage (C-V) measure-ments were carried out to determine the dielectric constant of thermally oxidized dielec-trics. We fabricated MoS2 field-effect transistors (FETs) with thermally oxidized ultra-thin and ultra-smooth Ta2O5 as top-gate and bottom-gate high-κ dielectric layers. We observed promis-ing device performance, including a nearly ideal subthreshold swing of ~ 61 mV/dec at room temperature, negligible hysteresis, drain-current saturation in the output characteristics, a high on/off ratio ~ 106, and a room temperature field-effect mobility exceeding 60 cm2/Vs. To fur-ther reduce the leak current and improve the device performance, we have also investigated the chemical transformation of HfSe2 to HfO2 high-κ dielectric, which has significantly larger band gap than Ta2O5

Chemical Vapor Deposition Growth of Two-Dimensional Transition Metal Dichalcogenides and Related Heterostructures

University of Minnesota Ph.D. dissertation. August 2018. Major: Chemistry. Advisor: James Johns. 1 computer file (PDF); xvii, 172 pages.Two-dimensional (2D) transition metal dichalcogenides (TMDCs) are atomically thin, layered materials with unique physical and electronic properties relative to their bulk forms. Due to these properties, 2D TMDCs show promise for many applications, including catalysis, nanoelectronics, optoelectronics, and spin- and valleytronics. To utilize TMDCs for these applications, they must first be reproducibly isolated. Much previous work in this area has resulted in material batches with low yield, small crystal sizes, and little control over the crystal morphology and orientation. Here, I present the reproducible chemical vapor deposition (CVD) growth of a wide array of 2D TMDCs, including MoS2, WS2, MoTe2, NbS2, and WSe2. Control of the growth of these materials is achieved through the optimization of many parameters, including substrate surface chemistry and synthetic growth parameters. Through the optimization of these parameters, I demonstrate control over the resulting material thickness, phase, and morphology. These high-quality TMDCs are subsequently used to grow many relevant heterostructures, including MoS2/WS2 lateral and vertical heterostructures, MoO2/MoS2 core/shell plates, 2H-1T´ MoTe2 few-layer homojunctions, and WS2/NbS2 lateral heterostructures, and the utility of these heterostructures is assessed. MoS2/WS2 heterostructures show promise as a semiconductor-semiconductor heterostructure in which the nature of the alignment is controlled by the initial MoS2 seed crystal. MoO2/MoS2 core/shell plates are freestanding and show epitaxial alignment with the underlying crystal substrate, with potential applications in catalysis. 2H-1T´ MoTe2 few-layer homojunctions are grown using a patternable phase engineering procedure, and devices fabricated from these homojunctions show reduced contact resistance relative to 2H MoTe2 devices with noble metal contacts. Finally, WS2/NbS2 lateral heterostructures show promise as an alternative metal-semiconductor heterostructure system for creating 2D TMDC devices with low contact resistance. The controlled CVD growth of these materials and heterostructures bolsters their future use for relevant applications

ATOMIC LAYER DEPOSITION OF SOLID ELECTROLYTES FOR BEYOND LITHIUM-ION BATTERIES

This thesis outlines methodology and development of an atomic layer deposition (ALD) process for the well-known solid-state electrolyte lithium phosphorous oxynitride (LiPON). I have developed a quaternary ALD LiPON process through a novel stepwise additive development procedure. ALD process kinetics and chemistry were investigated using in-operando¬ spectroscopic ellipsometry and in-situ x-ray photoelectron spectroscopy (XPS). ALD LiPON exhibits a tunable ionic conductivity proportional to N content, with the highest conductivity of 6.5x10-7 S/cm at 16.3% N. Two applications of ALD LiPON are investigated: ALD LiPON films as a protection layer for next-generation lithium metal anodes in the lithium sulfur battery system, and as solid electrolytes in 3D thin film batteries with discussion towards development of an all ALD 3D battery. Lithium metal is considered the “holy grail” of battery anodes for beyond Li-ion technologies, however, the high reactivity of Li metal has until now prevented its commercial use. Here, ALD protection layers are applied directly to the Li anode to prevent chemical breakdown of the liquid electrolytes while allowing ion transport through the protection layer. Protection of lithium metal is investigated with two materials: low ionic conductivity ALD Al2O3, demonstrating a 60% capacity improvement in Li-S batteries by protecting the Li anode from sulfur corrosion during cycling, and high ionic conductivity ALD LiPON, demonstrating a 600% improvement in Li-S battery capacity over unprotected anodes. Interestingly, ALD LiPON also forms a self-healing protection layer on the anode surface preventing deleterious Li dendrite formation during high rate cycling. Solid Li-based inorganic electrolytes offer two profound advantages for energy storage in 3-D solid state batteries: enhanced safety, and high power and energy density. Until now, conventional solid electrolyte deposition techniques have faced hurdles to successfully fabricate devices on challenging high aspect ratio structures, required for improvements in both device energy and power density. In this thesis, I demonstrate fabrication of ALD heterostructures suitable for use in 3D solid batteries, and although this work is incomplete I discuss progress towards future use of ALD LiPON solid electrolytes in all ALD solid-state 3D batteries