Nature of intrinsic and extrinsic electron trapping in SiO 2

Using classical and ab initio calculations we demonstrate that extra electrons can be trapped in pure crystalline and amorphous SiO2 (a-SiO2) in deep band gap states. The structure of trapped electron sites in pure a-SiO2 is similar to that of Ge electron centers and so-called [SiO4/Li]0 centers in α quartz. Classical potentials were used to generate amorphous silica models and density functional theory to characterize the geometrical and electronic structures of trapped electrons in crystalline and amorphous silica. The calculations demonstrate that an extra electron can be trapped at a Ge impurity in α quartz in six different configurations. An electron in the [SiO4/Li]0 center is trapped on a regular Si ion with the Li ion residing nearby. Extra electrons can trap spontaneously on pre-existing structural precursors in amorphous SiO2, while the electron self-trapping in α quartz requires overcoming a barrier of about 0.6 eV. The precursors for electron trapping in amorphous SiO2 comprise wide (≥132∘) O–Si–O angles and elongated Si–O bonds at the tails of corresponding distributions. Using this criterion, we estimate the concentration of these electron trapping sites at ≈4×1019 cm−3

Nitrogen acceptor in 2H-polytyp synthetic MoS2 assessed by multifrequency electron spin resonance

© 2018 Published by the AVS. Electron spin resonance (ESR) study on 2H-polytype synthetic MoS2 revealed the N acceptor dopants as being characterized by a spectrum of axial symmetry [g∥ = 2.032(2); g⊥ = 2.270(2)], typical for a hole-type center in MoS2. The N impurities substitute for S sites, with a density of ∼2.3 × 1017 cm−3, which accounts for the overall p-type doping. With respect to measurements for the applied magnetic field directed along the c-axis, the signal consists of a 14N primary hyperfine triplet of splitting constant A∥ = 14.7 ± 0.2 G superimposed on a correlated Gaussian single central line of peak-to-peak width ΔBpp = 15.3 ± 0.5 G, the latter making up only ∼26% of the total signal intensity. The current work extends on these results through extensive monitoring of the temperature (T) dependence of salient ESR parameters and studying the impact of thermal treatment. ESR signal saturation studies indicate a N acceptor spin-lattice relaxation time T1 (4.2 K) ≈ 3 × 10−4 s, notably different from the much smaller As acceptor’s T1 in geological MoS2. Concerning the thermal stability of the dopant, the N acceptor is found to be drastically passivated when exposed to H2 at ∼500 °C. Yet, subsequent reactivation attempts in vacuum at temperatures up to 740 °C appear unsuccessful, urging great caution with conventional forming gas treatments at T ≳ 500 °C. Combination of careful K- and Q-band ESR monitoring of the T-dependent signal intensity resulted in the consolidation of the N dopant as a shallow acceptor of activation energy Ea = 45 ± 7 meV. The consolidated results establish N as a promising candidate for stable covalently bonded p-type doping of MoS2 layers intended for application in novel nanoelectronic devices.status: publishe

Hydrogen induced dipole at the Pt/oxide interface in MOS devices

Thanks to its good thermal stability, including resistance to oxidation, platinum (Pt) is widely used in prototyping a wide spectrum of electron devices ranging from metaloxide- semiconductor (MOS) transistors to resistive switching memory cells. In this work, the energy barriers for electrons between the Fermi level of Pt and the conduction band of several oxide insulators (SiO2, Al2 O3, HfO2, Hf0.8Al0.2Ox, Sr0.53Ti0.47O3) were determined by using internal photoemission of electrons. By combining this barrier value with the electron affinity of the particular oxide, the effective work function (EWF) of Pt was determined for different interfaces. As studied over the reference Pt/oxide/Si stacks de-gassed in high vacuum at 400 8C, the EWF of Pt is found to differ significantly from the accepted vacuum WF value of 5.6 eV. The EWF is equal to 5.2 eV at the Pt/Al2O3 interface, 5.1 eV at Pt/HfO2, 5.3 eV at Pt/Hf0.8Al0.2Ox, 4.8 eV at Pt/SiO2, and 5.8 eV at the Pt/Sr0.53Ti0.47O3 interface indicating the presence of a polarization layer of which the contribution to the EWF depends on the oxide composition. Furthermore, annealing in H2 at 400 8C reduces the Pt EWF by 0.5 eV at all interfaces except for the Pt/Sr0.53Ti0.47O3 one. This observation indicates the formation of an additional H-related dipole at the Pt/oxide interfaces and suggests that the vacuum WF of Pt cannot be used as the value relevant for the MOS properties.status: publishe

Band alignment at interfaces of two-dimensional materials: internal photoemission analysis

The article overviews experimental results obtained by applying internal photoemission (IPE) spectroscopy methods to characterize electron states in single- or few-monolayer thick two-dimensional materials and at their interfaces. Several conducting (graphene) and semiconducting (transitional metal dichalcogenides MoS2, WS2, MoSe2, and WSe2) films on top of thermal SiO2 have been analyzed by IPE, which reveals significant sensitivity of interface band offsets and barriers to the details of the material and interface fabrication, indicating violation of the Schottky-Mott rule. This variability is associated with charges and dipoles formed at the interfaces with van der Waals bonding as opposed to the chemically bonded interfaces of three-dimensional semiconductors and metals. Chemical modification of the underlying SiO2 surface is shown to be a significant factor, affecting interface barriers due to violation of the interface electroneutrality.status: publishe

High-k dielectrics and metal gates for future generation memory devices

The requirements and development of high-k dielectric films for application in storage cells of future generation flash and Dynamic Random Access Memory (DRAM) devices are reviewed. Dielectrics with k-value in the 9-30 range are studied as insulators between charge storage layers and control gates in flash devices. For this application, large band gaps (> 6 eV) and band offsets are required, as well as low trap densities. Materials studied include aluminates and scandates. For DRAM metal-insulator-metal (MIM) capacitors, aggressive scaling of the equivalent oxide thickness (with targets down to 0.3 nm) drives the research towards dielectrics with k-values > 50. Due to the high aspect ratio of MIMCap structures, highly conformal deposition techniques are needed, triggering a substantial effort to develop Atomic Layer Deposition (ALD) processes for the deposition of metal gates and high-k dielectrics. Materials studied include Sr and Ba-based perovskites, with SrTiO3 as one of the most promising candidates, as well as tantalates, titanates and niobates