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A thorough understanding of native oxides is essential for designing semiconductor devices. Here we report a study of the rate and mechanisms of spontaneous oxidation of bulk single crystals of ZrS$_x$Se$_{2-x}$ alloys and MoS$_2$. ZrS$_x$Se$_{2-x}$ alloys oxidize rapidly, and the oxidation rate increases with Se content. Oxidation of basal surfaces is initiated by favorable O$_2$ adsorption and proceeds by a mechanism of Zr-O bond switching, that collapses the van der Waals gaps, and is facilitated by progressive redox transitions of the chalcogen. The rate-limiting process is the formation and out-diffusion of SO$_2$. In contrast, MoS$_2$ basal surfaces are stable due to unfavorable oxygen adsorption. Our results provide insight and quantitative guidance for designing and processing semiconductor devices based on ZrS$_x$Se$_{2-x}$ and MoS$_2$, and identify the atomistic-scale mechanisms of bonding and phase transformations in layered materials with competing anions.

ZrS2, ZrSe2 and mixed alloy ZrSxSe2−x materials were achieved through chemical vapor transport. The incongruent melting system of Zr-S-Se formed crystalline layered flakes as a transport product that grew up to 2 cm in lateral size with cm-scale flakes consistently obtained for the entire compositional range exhibiting visible hexagonal features. Bulk flakes of the series ZrSxSe2−x (x = 0, 0.15, 0.3, 0.6, 1.05, 1.14, 1.51, 1.8 and 2) were analyzed through Raman spectroscopy revealing significant convolution of primary bonding modes and shifting of Raman features as a function of increasing sulfur composition. Additionally, activation of new modes not present in the pure compounds are observed as effects which result from disorder introduced into the crystal due to the random mixing of S-Se in the alloying process. Further structural characterization was performed via x-ray diffraction (XRD) on the layered flakes to evaluate the progression of layer spacing function of alloy composition which was found to range between 6.24 A for ZrSe2 and 5.85 A for ZrS2. Estimation of the compositional ratios of the alloy flakes through energy dispersive spectroscopy (EDS) large-area mapping verified the relation of the targeted source stoichiometry represented in the layered flakes. Atomic-resolution high angle annular dark field (HAADF)-scanning transmission electron microscopy (STEM) imaging was performed on the representative Zr(S0.5Se0.5)2 alloy to validate the 1T atomic structure and observe the arrangement of the chalcogenide columns stacks. Additionally, selected area diffraction pattern generated from the [0 0 0 1] zone axis revealed the in-plane lattice parameter to be approximately 3.715 A.

We present a comprehensive study on the integration of hexagonal boron nitride (h-BN) with epitaxial graphene (EG) and bilayer hydrogen intercalated EG. Charged impurity scattering is the dominant scattering mechanism for as-grown and h-BN coated graphene. Use of h-BN dielectrics leads to a 2.6× improvement in Hall mobility relative to HfO2 by introducing less charged impurities and negligible additional remote surface optical scattering beyond that introduced by the substrate. Temperature dependent mobility measurement is used to link the surface morphology of the silicon carbide substrate (i.e., step-edge density) with charge carrier transport, showing that significant degradation in mobility can result from increased remote charged impurity as well as remote surface optical scattering at the SiC step-edges. Furthermore, we demonstrate that the integration of h-BN with EG and bilayer graphene presents unique challenges compared to previous works on exfoliated graphene, where the benefits of h-BN as a dielectric is highly dependent on the initial quality of the EG. To this end, modeling of the carrier mobility as a function of impurity density is used to identify the regimes where h-BN dielectrics outperform conventional dielectrics and where they fail to surpass them. Modeling indicates that h-BN can ultimately lead to a >5× increase in mobility relative to HfO2 dielectrics due to higher energy surface optical phonon (SOP) modes.

The effects of growth conditions, diffusion barrier coatings, and hot zone materials on B incorporation in 6H-SiC crystals grown by physical vapor transport (PVT) were evaluated. Development of high purity source material with a B concentration less than 1.8x1015 atoms/cm3, was critical to the growth of boules with a B concentration less than 3.0x1016 atoms/cm3. Application of refractory metal carbide coatings to commercial graphite to serve as boron diffusion barriers and the use of very high purity pyrolytic graphite components ultimately led to the growth of SiC boules with boron concentrations as low as 2.4x1015 atoms/cm3. The effect of growth temperature and pressure were closely examined over a range from 2100°C to 2300°C and 5 to 13.5 Torr. This range of growth conditions and growth rates had no effect on B incorporation. Attempts to alter the gas phase stoichiometry through addition of hydrogen gas to the growth environment also had no impact on B incorporation. These results are explained by considering site competition effects and the ability of B to diffuse through the graphite growth cell components.

The effects of growth conditions, diffusion barrier coatings, and alternate hot zone materials on boron incorporation in 6H-SiC boules grown by physical vapor transport were evaluated. Chemical analysis by secondary ion mass spectrometry revealed that commercial source materials combined with standard halogen-purified graphite led to B concentrations on the order of 1.0e17 atoms/cm 3 . Development of high-purity source material with B concentrations less than 1.8e15 atoms/cm 3 subsequently reduced the B concentration in the boule to 3.0e16 atoms/cm 3 . Application of carbide coatings and the use of pyrolytic graphite components ultimately lead to the growth of SiC boules with B concentrations as low as 2.4e15 atoms/cm 3 . Changes in growth temperature (2100–2300 °C) and pressure (5–13.5 Torr) had no measurable effect on B incorporation. Changes in the C/Si ratio due to source graphitization and addition of hydrogen gas to the growth atmosphere also had no effect on B incorporation.

A key limitation to graphene based electronics is graphene’s interaction with dielectric interfaces. SiO 2 and various high-k gate dielectrics can introduce scattering from charged surface states, impurities, and surface optical phonons; degrading the transport properties of graphene. Hexagonal boron nitride (h-BN) exhibits an atomically smooth surface that is expected to be free of dangling bonds, leading to an interface that is relatively free of surface charge traps and adsorbed impurities. Additionally, the decreased surface optical phonon interaction from h-BN is expected to further reduce scattering. While h-BN gated graphene FETs have been demonstrated on a small scale utilizing CVD grown or exfoliated graphene, integrating quasi-freestanding epitaxial graphene (QFEG) with h-BN gate dielectrics on a wafer scale has not been explored. We present results from the first large scale CVD growth of h-BN and its subsequent transfer to a 75mm QFEG wafer. The effects of growth conditions on the thickness and quality of the h-BN film and its potential and limitations as a gate dielectric to QFEG are discussed. The introduction of charged impurities during the transfer process resulted in an average degradation in mobility of only 9%. Despite the slight degradation, we show that h-BN is highly beneficial compared to high-k dielectrics when the charged impurity concentration of QFEG is below 5x10 12 cm -2 . Here we show improvements in mobility of >3x and intrinsic cutoff frequency of >2x compared to HfO 2 .

In recent years, hexagonal boron nitride (h-BN) has gained interest as a material for use in graphene based electronics, where its ultra-smooth two-dimensional structure, lack of dangling bonds, and high energy surface optical phonon modes are desirable when considering the effect of dielectric materials in introducing additional sources of scattering for carriers within graphene. Initial work has indicated that use of h-BN in place of SiO 2 supporting substrates can lead to 2–3x improvements in device performance [1,2], suggesting that h-BN may be an excellent choice as top-gate dielectric for graphene devices. In this work, we integrate h-BN with quasi-freestanding graphene (QFEG) for the first time and demonstrate a 2x improvement in radio frequency (RF) performance and the highest ƒ T ·L g product yet reported for h-BN integrated graphene devices (25 GHz·µm).

Hexagonal boron nitride (h-BN) is a promising dielectric material for graphene-based electronic devices. Here we investigate the potential of h-BN gate dielectrics, grown by chemical vapor deposition (CVD), for integration with quasi-freestanding epitaxial graphene (QFEG). We discuss the large scale growth of h-BN on copper foil via a catalytic thermal CVD process and the subsequent transfer of h-BN to a 75 mm QFEG wafer. X-ray photoelectron spectroscopy (XPS) measurements confirm the absence of h-BN/graphitic domains and indicate that the film is chemically stable throughout the transfer process, while Raman spectroscopy indicates a 42% relaxation of compressive stress following removal of the copper substrate and subsequent transfer of h-BN to QFEG. Despite stress-induced wrinkling observed in the films, Hall effect measurements show little degradation (10%) in carrier mobility for h-BN coated QFEG. Temperature dependent Hall measurements indicate little contribution from remote surface optical phonon scattering and suggest that, compared to HfO(2) based dielectrics, h-BN can be an excellent material for preserving electrical transport properties. Graphene transistors utilizing h-BN gates exhibit peak intrinsic cutoff frequencies30 GHz (2.4× that of HfO(2)-based devices).

We explore the effect of high-κ dielectric seed layer and overlayer on carrier transport in epitaxial graphene. We introduce a novel seeding technique for depositing dielectrics by atomic layer deposition that utilizes direct deposition of high-κ seed layers and can lead to an increase in Hall mobility up to 70% from as-grown. Additionally, high-κ seeded dielectrics are shown to produce superior transistor performance relative to low-κ seeded dielectrics and the presence of heterogeneous seed/overlayer structures is found to be detrimental to transistor performance, reducing effective mobility by 30-40%. The direct deposition of high-purity oxide seed represents the first robust method for the deposition of uniform atomic layer deposited dielectrics on epitaxial graphene that improves carrier transport.