Silicon Inversion Layers Research Articles

Simulation study on lateral minority carrier transport in the surface inversion layer of the p-aSi:H/i-aSi:H/cSi heterojunction solar cell

The theoretical lateral current of the surface inversion layer in a crystalline silicon (cSi) surface for a p-aSi:H/i-aSi:H/cSi heterojunction (SHJ) solar cell was calculated using computer simulation and was compared with the experimental one to study defects/traps at the aSi:H/cSi interface and/or in the cSi surface and to detect the acceptor concentration (N a) in p-aSi:H. To experimentally extract the lateral surface inversion layer current, a field-effect transistor type test element group device was co-integrated with SHJ cells on the same wafer. From the correlation between the experimental and calculated lateral surface inversion layer current, the density of defects/traps (D it) at the aSi:H/cSi interface and/or in the cSi surface and the value of N a were extracted. The calculated lateral surface inversion layer current stayed unchanged for various minority carrier lifetimes in the substrate, suggesting that this method is not suffered from the variation in the material parameters in the substrate.

Performance enhancement of organic/Si solar cell using CNT embedded hole selective layer

In this work, we present a simple and solution-based synthesis approach to combine the virtues of PEDOT:PSS and carbon nanotube (CNT) by developing a composite film and their application as a hole selective layer (HSL) in organic-silicon hybrid heterojunction solar cell. The heterojunction solar cell with the optimised PEDOT: PSS-CNT HSL showed a notable open-circuit voltage (Voc) of 588.6 mV, short circuit current density (Jsc) of 25.3 mA/cm2 and fill factor (FF) of 60.79% yielding a photo-conversion efficiency (η) of 9.05%. The pristine PEDOT:PSS/Si device obtained an η of 6.2% with a Voc of 502 mV. The role of CNT in the charge transport mechanism and improving Voc was corroborated by employing various characterisation methods such as J-V, C-V, and electrochemical impedance spectroscopy. The composite layer induced a strong inversion layer in silicon and enhanced the device performance. The application of PEDOT:PSS-CNT composite film is not limited to c-Si solar cells, but can also be used to fabricate flexible organic and perovskite solar cells.

Surface inversion layer effective minority carrier mobility as one of the measures of surface quality of the p-aSi:H/i-aSi:H/cSi heterojunction solar cell

The lateral transport of minority carriers in the surface inversion layer of a crystalline silicon (cSi) surface for a p-aSi:H/i-aSi:H/cSi heterojunction (SHJ) solar cell was experimentally analyzed to study defects/traps at the aSi:H/cSi interface and/or on the cSi surface. To extract the lateral surface inversion layer current, a field-effect transistor type test element group device was designed and fabricated on a cSi wafer where SHJ cells were co-integrated. By analyzing the lateral surface inversion layer current, the effective surface minority carrier mobility was calculated. The extracted mobility was one order of magnitude smaller than that of a reference MoOx/i-aSi:H/cSi structure but it increased by low-temperature post-deposition annealing with respect to the reference. This method is highly sensitive to the quality of the aSi:H/cSi interface and/or the cSi surface. The density of trap sites was estimated to be of the order of 1011 cm−2 at the aSi:H/cSi interface and/or on the cSi surface.

An analytical model for the electron effective mobility in a strained silicon inversion layer

An electron effective mobility analytical model without empirical parameters is investigated for the strained silicon inversion layer, which can be conveniently applied by device and circuit designers. Four kinds of scattering mechanisms, i.e., coulomb scattering, acoustic phonon scattering, intervalley phonon scattering and surface roughness scattering, are taken into account to calculate the electron mobility for the 2D (two-dimensional) inversion layer through a one-dimensional inverse transform. Considering the quantum confinement and strain effect, the valley electron occupancy variation is dissected. In regard to the Si/(001)Si1−xGex and Si/(110)Si1−xGex Si common orientation, the dependence of the electron effective mobility with various Ge content on the inversion charge density is analyzed in detail, and by means of this the mechanism of the mobility enhancement under different situations and the mobility saturation are brought to light.

Low Field Electron Mobility Degradation in Silicon MOS Transistors Under Uniform Hot Electron Injection

Low field mobility of electrons in inversion layers of silicon ${n}$ -type MOS transistors exposed to uniform (non-avalanche) hot electron injection is studied at ambient temperature. Mobility degradation by Coulomb scattering with stress induced charged states at the silicon/insulator interface and in the insulator bulk is investigated and interpreted on the basis of a numerical model. The model accounts for multi-subband transport and a detailed description of Coulomb scattering with charged centers including non-local screening by free-electrons in different subbands. Calculations are shown to be in good agreement with measured data. Exploiting Matthiessen’s rule, mobility limited by Coulomb scattering with (stress induced) interface and insulator charges is extracted from measurements before and after stress and compared with calculations. The validity of Matthiessen’s rule is discussed. Finally, a simple local model for mobility limited by Coulomb scattering with charged interface states, suitable for implementation in a commercial TCAD simulator, is proposed.

Ultraviolet Characteristics of a Silicon Inversion Layer for Applications in Radiometry

Ultraviolet Characteristics of a Silicon Inversion Layer for Applications in Radiometry

Physical insight into reduced surface roughness scattering in strained silicon inversion layers

A seemingly anomalous enhancement of electron mobility in strained silicon inversion layers at high sheet densities has exposed a conspicuous gap between device physics theory and experiment in recent years. We show that the root of this discrepancy is due to a bulging effect in the electron \Delta 4 wavefunction at the silicon surface. This renders \Delta 4 electrons more susceptible to perturbations in surface structure thereby increasing surface roughness scattering for these states. Strain engineering utilized by the CMOS industry reduces the relative occupancy of the \Delta 4 states resulting in less overall surface roughness scattering in the channel. We show that the origin of this effect can be explained by moving beyond the effective mass approximation and contrasting the properties of the \Delta 2 and \Delta 4 wavefunctions in a representation that comprehends full crystal and Bloch state symmetry.

Hole effective mass in silicon inversion layers with different substrate orientations and channel directions

We explore the possibility to define an effective mass parameter to describe hole transport in inversion layers in bulk MOSFETs and silicon-on-insulator devices. To do so, we employ an accurate and computationally efficient self-consistent simulator based on the six-band k·p model. The valence band structure is computed for different substrate orientations and silicon layer thicknesses and is then characterized through the calculation of different effective masses taking account of the channel direction. The effective masses for quantization and density of states are extracted from the computed energy levels and subband populations, respectively. For the transport mass, a weighted averaging procedure is introduced and justified by comparing the results with hole mobility from experiments and simulations.

Dipole trap model for the metal-insulator transition in gated silicon-inversion layers

In order to investigate the metallic state in high-mobility Si-MOS structures, we have further developed and precised the dipole trap model which was originally proposed by B.L. Altshuler and D.L. Maslov [Phys. Rev. Lett.\ 82, 145 (1999)]. Our additional numerical treatment enables us to drop several approximations and to introduce a limited spatial depth of the trap states inside the oxide as well as to include a distribution of trap energies. It turns out that a pronounced metallic state can be caused by such trap states at appropriate energies whose behavior is in good agreement with experimental observations.

Correlation-induced phase transitions in two-dimensional electron systems

Abstract : The electrons in inversion or accumulation layers of MOSFETs or in semiconductor-semiconductor interfaces show prominent many-body effects. A new theory off the correlation energy is developed for these electrons such that the results are applicable to a wide density range. The ground-state energy is obtained for all densities. It is continuous and convergent, but changes its analytical form at r sub s = 1.414. Associated with this change is a divergence of the compressibility that occurs at r sub s =1.989. Hence, around this r sub s, the system can be considered to be in a liquidlike state. In the (001) direction of silicon inversion layers, the two valleys, occupied equally by the electrons at high densities, may be populated unevenly due to electron correlations. This unbalance will cause a valley occupancy phase transition at r sub s =8.011, in close agreement with a recent experiment. Under a magnetic field, the susceptibility is enhanced nonlinearly when the density of electrons is reduced. This enhancement becomes very strong toward r sub s =13.0, beyond which a spin-polarized state is favored.

Extraction of Effective Carrier Velocity and Observation of Velocity Overshoot in Sub-40 nm MOSFETs

Carrier velocity in the MOSFET channel is the main driving force for improved transistor performance with scaling. We report measurements of the drift velocity of electrons and holes in silicon inversion layers. A technique for extracting effective carrier velocity which is a more accurate extraction method based on the actual inversion charge measurement is used. This method gives more accurate result over the whole range of V<SUB>ds</SUB>, because it does not assume a linear approximation to obtain the inversion charge and it does not limit the range of applicable V<SUB>ds</SUB>. For a very short channel length device, the electron velocity overshoot is observed at room temperature in 37 ㎚ MOSFETs while no hole velocity overshoot is observed down to 36 ㎚. The electron velocity of short channel device was found to be strongly dependent on the longitudinal field.

Determination of the conduction band offset between hydrogenated amorphous silicon and crystalline silicon from surface inversion layer conductance measurements

The presence of an electron rich inversion layer in p-type crystalline silicon (c-Si) at the interface with n-type hydrogenated amorphous silicon (a-Si:H) is experimentally demonstrated from the coplanar conductance of (n) a-Si:H∕(p) c-Si structures, which is shown to be very sensitive to the band mismatch between the two semiconductors. The temperature dependence of the corresponding sheet electron density allows to determine with a very good precision the conduction band offset ΔEC between a-Si:H and c-Si:ΔEC=0.15±0.04eV.

Effect of strain on the electron effective mobility in biaxially strained silicon inversion layers: An experimental and theoretical analysis via atomic force microscopy measurements and Kubo-Greenwood mobility calculations

Recently, in order to explain the long-channel electron effective mobility at a high sheet carrier density in strained silicon channel transistors, it has been suggested by [M. V. Fischetti, F. Gamiz, and W. Hansch, J. Appl. Phys. 92, 7230 (2002)] that biaxial tensile strain should smooth the Si/SiO2 interface. To address this topic, the roughness properties of biaxial strained silicon-on-insulator (s-SOI) films are investigated by means of atomic force microscopy. Through in-depth statistical analysis of the digitalized surface profiles, the roughness parameters are extracted for unstrained and strained SOI films, with 0.8% biaxial tensile strain. Especially, it is found that strain significantly reduces the roughness amplitude. Then, mobility calculations in SOI and s-SOI inversion layers are performed in the framework of the Kubo-Greenwood formalism. The model accounts for the main scattering mechanisms that are dominant in the high electron density range, namely phonon and surface roughness. Special attention has been paid to the modeling of the latter by accounting for all the contributions of the potential which arise from the deformed rough interface, and by using a multisubband wavelength-dependent screening model. This model is then applied to study the influence of the surface morphology on the mobility in s-SOI inversion layers. In this context, the mobility gain between s-SOI and unstrained SOI layers is found to agree significantly better with experimental data if the strain-induced decrease of the roughness amplitude is taken into account.

Hole mobility in silicon inversion layers: Stress and surface orientation

Hole transport in the p-type metal-oxide-semiconductor field-effect-transistor (p-MOSFET) inversion layer under arbitrary stress, surface, and channel orientation is investigated by employing a six-band k∙p model and finite difference formalism. The piezoresistance coefficients are calculated and measured at stresses up to 300MPa via wafer-bending experiments for stresses of technological importance: uniaxial and biaxial stresses on (001) and (110) surface oriented p-MOSFETs with ⟨110⟩ and ⟨111⟩ channels. With good agreement in the measured and calculated small stress piezoresistance coefficients, k∙p calculations are used to give physical insights into hole mobility enhancement at large stress (∼3GPa). The results show that the maximum hole mobility is similar for (001)∕⟨110⟩, (110)∕⟨110⟩, and (110)∕⟨111⟩ p-MOSFETs under uniaxial stress, although the enhancement factor is different. Strong quantum confinement and a low density of states cause less stress-induced mobility enhancement for (110) p-MOSFETs. For (001) p-MOSFETs, the dominant factor for the improved hole mobility is reduced conductivity effective mass at small stress and lower phonon scattering rate at large stress.

A theoretical interpretation of magnetoresistance mobility in silicon inversion layers

The magnetoresistance technique has been introduced recently as a means of determining experimentally the mobility in bulk metal-oxide-semiconductor transistor and silicon-on-insulator devices. This technique does not require a precise determination of the channel length, and it also has the advantage of allowing mobility extraction when the application of other methods is problematic, notably with weak inversion and short device length. The magnetoresistance mobility extracted in this way is related but not identical to the normal effective drift mobility. In this work we simulate electron transport in the presence of a magnetic field for different device structures. The simulations allow us to study the conditions under which magnetoresistance mobility and effective mobility coincide, and to measure the difference, where it exists. We find that at low temperatures the two quantities coincide, while at room temperature a difference of more than 20% may appear. We take this to be a consequence of the energy difference between subbands and the resulting distribution of electrons among them. We then explain the fact that the two mobility measures coincide at low temperatures when only the ground subband is occupied by analyzing the energy dependence of the momentum relaxation time.

Weak localization and correlation effects in thin-film degenerate n-type silicon

The magnetoconductance of thin-film n-type silicon has been studied in order to provide insight into the low-temperature behavior of recent silicon-based semiconductor-metal hybrid structures. There is considerable interest in such structures as potential candidates for creating nonmagnetic read-head sensors for ultrahigh-density recording. The magnetoconductance of thin-film silicon was therefore analyzed as a function of magnetic field orientation at temperatures ranging from T=4.2 to 300 K. At low temperatures, the magnetoconductance consists of two components that are attributed to weak localization and correlation effects. Surprisingly, the thin-film transport properties behave two-dimensionally for the localization contribution, whereas the correlation contribution is isotropic. Similar two-dimensional localization behavior has previously been reported for silicon inversion layers but was not expected for silicon systems with a finite thickness. An analysis of the magnetoconductance as a function of magnetic field orientation has verified that the localization contribution depends only on the perpendicular to plane component of the field. The Hall coefficient was observed to vary with magnetic field, providing further evidence for a magnetoconductance governed by electron-electron interactions. Fitting of the data at T=4.2 K and for low magnetic fields provided values for the localization parameter α, the inelastic scattering time τi, and the two-dimensional effective electron screening constant F̃σ.

Physical modeling of hole mobility in silicon inversion layers under uniaxial stress

A physical model for hole mobility under either biaxial or uniaxial stress has been developed. The six-band k ⋅ p theory is used to obtain the bandstructure through stress-dependent Hamiltonian. The hole mobility in the silicon inversion layer is studied in details using Monte Carlo method. A numerically robust method has been applied to achieve self-consistent solution of Poisson’s and Schrodinger equations.

Electron mobility in silicon and germanium inversion layers: The role of remote phonon scattering

We calculate the electron mobility in Si and Ge inversion layers in single-gate metal-oxide-semiconductor field effect transistors. Scattering with bulk phonons, surface roughness and remote phonons is included in the mobility calculations. Various high-κ dielectric materials are considered for both Si and Ge substrates. Overall, Ge outperforms Si, but in general Ge is more affected by the use of high-κ dielectrics. HfO2 degrades the mobility substantially compared to SiO2 for Si substrates and may prohibitively degrade performance. HfO2 with Ge yields an improvement over Si with a mobility enhancement ≈3× at an electron sheet density of 1×1013 cm−3.

Laudatio for Gerhard Abstreiter

Laudatio for Gerhard Abstreiter

Laudatio for Gerhard Abstreiter

Laudatio for Gerhard Abstreiter