Remote Surface Roughness Research Articles

Electron Mobility due to Surface Roughness Scattering in Depleted GaAs Free-Standing Thin Ribbon

Abstract:Electron mobility limited by surface roughness scattering in free-standing GaAs thin ribbon with an internal parabolic quantum well caused by surface state is investigated in detail. Based on analyzing the parabolic quantum well including the energy subband level, wave function and the confined potential profile in the thin ribbon by solving Schrödinger and Poisson equations self-consistently, the electron mobility could be investigated. Conclusion indicates that remote surface roughness (RSR) of the thin ribbon will change the two dimensional electron gas (2DEG) mobility through the medium of barrier height fluctuation of the parabolic well in atomic scale. Calculation results reveal that the 2DEG mobility decreases with increasing roughness amplitude, which is characterized in terms of the surface roughness height and the roughness lateral size.

Remote surface roughness scattering in fully depleted silicon-on-insulator devices with high-κ/SiO2 gate stacks

We investigate remote surface roughness (RSR) scattering by the SiO2/HfO2 interface in Fully Depleted Silicon-on-Insulator devices using Non-Equilibrium Green's Functions. We show that the RSR mobility is controlled by cross-correlations between the surface roughness profiles at the Si/SiO2 and SiO2/HfO2 interfaces. Therefore, surface roughness and remote surface roughness cannot be modeled as two independent mechanisms. RSR tends to enhance the total mobility when the Si/SiO2 interface and SiO2 thickness profiles are correlated, and to decrease the total mobility when they are anti-correlated. We discuss the implications for the high-κ/Metal gate technologies.

Graphene induced remote surface scattering in graphene/AlGaN/GaN heterostructures

The mobilities of single-layer graphene combined with AlGaN/GaN heterostructures on two-dimensional electron gases in graphene/AlGaN/GaN double heterojunction are calculated. The impact of electron density in single-layer graphene is also studied. Remote surface roughness (RSR) and remote interfacial charge (RIC) scatterings are introduced into this heterostructure. The mobilities limited by RSR and RIC are an order of magnitude higher than that of interface roughness and misfit dislocation. This study contributes to designing structures for generation of higher electron mobility in graphene/AlGaN/GaN double heterojunction.

Mobility-limiting mechanisms in polar semiconductor heterostructures

The mechanisms controlling the carrier mobility of two-dimensional electron gases (2DEGs) in ultrathin polar semiconductor heterostructures, such as III–V nitrides, have been analyzed. InxAl1−xN/AlN/GaN heterostructures with different AlN layer thicknesses have been investigated. These structures can be considered a very good benchmark for the analyses of III–V nitrides, due to the possibility of modulating the strain by varying the In composition. In order to determine an estimate of the mobility, charged dislocation and remote surface roughness scattering lifetimes have been calculated. Atomic force microscopy and scanning tunneling microscopy analyses have been used to measure the parameters required for the lifetime calculation, such as surface roughness, correlation length and dislocation density, and the total mobility has thus been calculated without the need of any a priori assumptions on the values of these parameters. The mobility of InxAl1−xN/AlN/GaN heterostructures has been measured at room temperature and liquid nitrogen temperature by the Hall effect. A comparison between the calculated and the Hall-effect-measured mobilities allowed us to establish, without using any ad hoc assumptions or fitting parameters, that the remote surface roughness is the most effective factor in controlling the transport properties of 2DEGs in nitride-based heterostructures at low temperature.

Polarization-mediated remote surface roughness scattering in ultrathin barrier GaN high-electron mobility transistors

Ultrathin AlN/GaN heterojunctions are highly attractive for high-frequency transistor applications. In this work, remote surface roughness (RSR) scattering mediated by the high polarization is studied as a new scattering mechanism in such structures. In both depletion-mode and enhancement-mode devices with ultrathin AlN barriers, RSR scattering can be of the same order as polar optical phonons, the dominant scattering mechanism at room temperature. The study indicates that to achieve high-performance high-electron mobility transistors, the surface roughness during processing is as critical as maintaining a sharp heterojunction.

Effect of Remote-Surface-Roughness Scattering on Electron Mobility in MOSFETs with High-k Dielectrics

In this paper, we developed a model for remote surface roughness scattering (RSR)-limited electron mobility in the inversion layer of nMOSFETs with high-k dielectrics. A Numerical method is applied to calculate RSR-limited electron mobility. It has been demonstrated that the RSR-limited electron mobility is highly degraded in the high electric field region.

Effect of Remote Surface Roughness Scattering on Electron Mobility in MOSFETs with High-k Dielectrics

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Remote surface roughness effects on inversion electron density in nano-MOSFET

In this paper the remote surface roughness effects on channel electron density of nano MOSFET is discussed by solving the coupled Schrodinger equation and Poisson equation. The results demonstrate that the remote surface roughness could largely affect the density of inversion channel electron, and the change of electron density by this effect has been found to have approximately linear relationship to the remote surface roughness. This implies that the mobility degradation effect by remote surface roughness scattering can be partially erased for the on-state current.

Key scattering mechanisms for holes in strained SiGe/Ge/SiGe square quantum wells

We present a theory of the low-temperature transport of holes confined in the Ge strained channel of single-side modulation-doped SiGe/Ge/SiGe square quantum wells (QWs). Besides the well-known scattering mechanisms such as remote impurities and surface roughness, the theory includes misfit deformation potential. We prove that due to the effect from doping-induced band bending, the surface roughness and misfit deformation potential scatterings are considerably strengthened. Accordingly, these are found to be the key scattering mechanisms in the SiGe/Ge/SiGe system, which are still a subject under debate. Our theory can explain all recent experimental data about the transport properties of interest, namely, the carrier-density dependences of the hole mobility and the ratio of the transport to quantum lifetimes. Further, the calculated hole mobility in Ge strained QWs exhibits a special channel-width dependence with a sharp peak, which was observed but has not been explained so far.

Non-equilibrium dielectric response of high-K gate stacks in Si MOSFETS: application to SO interface phonon scattering

The problem of coupling between phonon and plasmon modes across the idealised gate stack of a semiconductor MOSFET with one or more high-K dielectrics has come under close scrutiny in the last few years. High-K dielectrics are a possible technology for achieving the same gate capacitance as with silicon dioxide but for a much thicker insulating region. The effect is to drastically reduce the gate leakage current which is driven by tunnelling. Unfortunately, the much higher values of the dielectric parameters leads to the emergence of soft optical phonons that give rise to strong electron-phonon scattering at the channel-dielectric interface. Even for perfectly flat interfaces with homogeneous dielectric layers the technical problem of determining the modes and scattering amplitudes for electron-phonon-plasmon interactions is formidable. Here the problem is briefly reviewed and an outline is presented of a computational scheme to describe the modelling of MOSFETs with realistic high-K gate stacks that are dominated by interface roughness a layer inhomogeneities. The long term objective is to build both ensemble Monte Carlo modelling schemes and non-equilibrium Green function schemes for this problem. Preliminary data based on the simplest version of the computational scheme leads to the conclusion that the inhomogeneities in the gate stack give rise to a inhomogeneous electron-SO phonon scattering amplitude across the channel where the amplitude is modulated by remote and near surface roughness and by the electron concentration and electron temperature across the channel. Polycrystallinity of the dielectric is shown to lead to strong device parameter variations across ensembles of devices. The presence of an interfacial silicon dioxide layer between channel and high-K dielectric leads to a decrease in SO phonon scattering rates and higher effective mobilities as a function of increasing interfacial layer thickness.

Monte Carlo study of remote Coulomb and remote surface roughness scattering in nanoscale Ge PMOSFETs with ultrathin high- κ dielectrics

In this work we perform full-band Monte Carlo simulations of nanoscale Ge bulk PMOSFETs with ultrathin (<2 nm) effective oxide thickness high- κ dielectrics and investigate the importance of remote Coulomb and remote surface roughness scattering in these devices. In addition to these remote scattering mechanisms, transport is considered in the presence of phonon, ionized impurity, surface roughness scattering and impact ionization. Quantum confinement in the inversion layer is taken into account in the form of a modified potential. We show that the experimentally observed data on the dependence of mobility on the thickness of the high- κ dielectric can be accurately modeled via scattering by remote surface roughness at the gate-dielectric interface and remote Coulomb scattering due to fixed charges in the dielectric. The parameters in the remote scattering mechanisms are fitted by matching Monte Carlo data with those obtained from experiments on the dependence of mobility on the thickness of the dielectric. For a gate overdrive, ( V g − V t), of 1.0 V, remote scattering mechanisms seem to decrease the saturation current by as much as 15% in these devices. We also show that at the high vertical fields (∼1 MV/cm) in these experimental devices, remote scattering mechanisms can substantially reduce the mobility for oxide thicknesses below 5 nm.

Effects of remote-surface-roughness scattering on carrier mobility in field-effect-transistors with ultrathin gate dielectrics

We examined effects of the remote surface roughness, which is the roughness between the polycrystalline silicon gate and gate dielectric, on the inversion carrier mobility of metal-insulator-semiconductor field-effect-transistors with ultrathin gate dielectrics. We calculated the effective mobility by the linear response theory and found that the scattering from the remote surface roughness reduces the effective mobility especially at high vertical fields. The effective mobility is severely reduced, if the correlation length of the remote surface roughness is comparable to the inverse of thermal de Broglie wave number. We show that the hole mobility reduction experimentally found for the transistor with the Al2O3 gate dielectric can be explained by this scattering.

Scattering of electrons in silicon inversion layers by remote surface roughness

A model to study the effect of the roughness at the poly-Si/SiO2 interface in silicon inversion layers on the electron mobility is obtained. Screening of the resulting perturbation potential by the channel carriers is taken into account, considering Green’s functions for metal–oxide–semiconductor geometry, i.e., taking into account the finite thickness of the gate oxide. Mobility of electrons is evaluated at room temperature by the Monte Carlo method, taking into account the simultaneous contribution of phonon scattering, SiO2/Si interface roughness scattering, Coulomb scattering, and remote surface roughness scattering. The contribution of excited subbands is considered. The resulting remote surface roughness scattering is shown to be strongly dependent on the oxide thickness, and degrades mobility curves at low inversion charge concentrations. The results obtained show that the effect of this scattering mechanism cannot be ignored when the oxide thickness is below 5 nm, (as in actual devices), even when (as is usual) very high doping concentrations are used.

Laser diffraction method of surface roughness measurement

Traditional contact probe and stylus methods have been difficult to apply for the measurement of the roughness of sculptured surfaces, especially whose positioned at under high inclination. The laser diffraction method is presented as a method for remote surface roughness measurement that can be used directly on a machine tool or a press for any position of complex sculptured surfaces. This method can be used to measure the surface roughness of various materials, both metals and non-metals, as well as hard ceramic and soft plastic such as for rapid prototyping. The main technical difference of the method suggested is the use of a new optical system and the automatic determination of the luminous radiation reflection coefficient by a proposed analytical dependence. Specifically, the method involves the simultaneous measurement of the mirror reflected radiation beams as well as the scattered spectrum component and then the numerical processing of reflected laser light signal by developed software. The measurement results taken for different machining processes of complex sculptured surfaces have shown a good correlation with the results of stylus methods.

The role of interfaces in ultrasmall semiconductor devices

As semiconductor devices become smaller, the temporal and spatial scales become sufficiently short and the electric field sufficiently large that new physical constraints are reached in treating the transport of carriers within the devices. These devices are now controlled by strong size-related effects: coupling to the environment of contacts, interfaces/boundaries/surfaces, interconnects, and other devices. This is especially the case in MOSFET’s, where remote interface phonons and surface roughness scattering dominate the transport. In this talk, a general treatment of device–environment interaction will be discussed. Special cases for MOS transport and the specific role of interfaces will be treated.