Evidence for mobility domains in (100) silicon inversion layers

In proof of the existence of domains with different mobilities measurements of Shubnikov-de Haas oscillations, piezoresistance, and transverse “Hall” field due to mobility anisotropy are provided. The energy splitting between the ground state subbands of different valleys has been determined.     

Zeros of the transverse magneto-resistance in N-type silicon (100) inversion layers

The transverse and Hall resistance are investigated under quantizing electric and magnetic fields in n-type silicon (100) MOSFET inversion layers. The transverse resistance ρ_xx vanishes within finite ranges of the gate voltage where the concentration of channel conduction electrons is constant. The variation of the oscillatory period towards low gate voltages is not compatible with the concept of carrier localization and therefore can be understood by charge transfer into states outside the surface channel.     

Effect of neutral scatterers on electron mobility in silicon (100) inversion layers

The electron mobility inn-type inversion layers at (100) silicon surface is studied at low temperature. Using the lowest subband calculated numerically, and taking into account the neutral impurity distribution in the vicinity of the Si–SiO₂ interface, the process of carrier scattering is investigated by the Green's function formalism. Experimental data for electron mobility are in good agreement with the results from Dyson's equation in Born approximation in the case of shallow and steep profiles of impurities.     

Temperature dependence of the conductivity of a silicon inversion layer at low temperatures

The temperature dependence of the conductivity of a high-mobility silicon MOSFET is reported for temperatures from 0.2 to 23 K and electron densities from 1 to 20 × 10¹¹cm⁻². The results show a strong increase in conductivity with decreasing temperature. However, this rate of increase was observed to rapidly diminish outside of a restricted range of temperature for a given density. The decreasing temperature dependence at the lower temperatures is attributed to the saturation of screening as the effects of broadening become comparable to thermal effects. The diminishing temperature dependence at the higher temperatures puts limits on the applicability of recent calculations for the conductivity.     

Magneto-transport properties of silicon inversion layers at low carrier densities

Conductivity σ and magneto-resistance of n- and p-channel MOSFET's with low oxide-charge density were investigated between 1.2 K and 4.2 K and carrier concentrations between 10¹¹/cm² and 10¹²/cm². Data were taken at different source drain electric field strengths. At small source drain fields and low carrier concentrations σ showed an activated behaviour with the activation energy decreasing with increasing carrier concentration. At source drain fields of the order 1 V/cm pronounced deviations from the activated behaviour were observed. In addition, the channel current-voltage characteristics of the devices studied were nonlinear. Shubnikov-De Haas oscillations were recorded at surface carrier concentrations as low as 10¹¹/cm². The data cannot be explained with a hopping model or by the existence of a mobility edge.     

Surface quantum oscillations in n-type (100) silicon inversion layers on sapphire

Measurements of Shubnikov-de Haas oscillations in (100) Si/($\text{1}$012)Al₂O₃ MOSFET's were performed in magnetic fields up to 10 Tesla for different tilt angles between the magnetic field direction and the surface normal. The experimental results show that the lowest electric subband in this system is twofold degenerate and is formed by the “heavy” cyclotron mass valleys. This can be explained by a large lateral stress present in the SOS (silicon on sapphire) system.     

Quantum oscillations in p-type inversion layers of (111) and (100) silicon field effect transistors

For the first time Shubnikov-de Haas oscillations have been observed in p-type inversion layers of (111) and (100) silicon field effect transistors. For both orientations a single electric subband was found. The effective mass of the surface carriers was determined from the temperature dependence of the oscillations, for (111) surfaces as a function of the surface electric field. At low gate voltages spin splitting of the Landau levels was resolved.     

Transport properties of conduction electrons in n-type inversion layers in (100) surfaces of silicon

The conductivities of n-type inversion layers in (100) surfaces of p-type silicon were measured extensively as functions of electron density in the inversion layer, the ambient temperature and the applied magnetic field. Measurements were made on the carefully fabricated four “classes” of MOS field-effect transistors whose maximum mobilities at 4·2K were 14,000, 8000, 6800 and 1500 cm²/V·sec, respectively. From the temperature dependence of the mobility, dominant momentum scattering was reasonably ascribed to surfon at 100 ～ 300 K. and degenerate or non-degenerate coulomb scattering at lower temperatures as treated by Stern and Howard. From the curves of conductivity vs temperature at low temperatures and low electron concentration for specimens with high mobilities, an activation energy of 1·2 meV, relating to the shallow bound states associated with the lowest electrin sub-band, was observed. The conductivity σ_xx of the inversion layer in a strong transverse magnetic field showed behaviors like those of completely free electrons without effects belonging to its material in its oscillation pattern. That is, the peak value of σ_xx as a function of the gate voltage V_R dependend only on the Landau index. The σ_xx as a function of the magnetic field H at a constant V_R showed a similar Shubnikov-de Haas (SdH) type oscillation to that of three dimensional one. The SdH oscillation gave an “apparent” g-value g* which ranges from 2 to 5 depending on the surface carrier density n_s, due to the change in the ratios of the widths of the Landau levels to the level separation. The “reasonable” g-value of the conduction electrons in the inversion layer has been determined using a modified tilted magnetic field method. The g-value at the fixed magnetic field was independent of surface carrier density n_s and tended to 2 in the extreme strong magnetic field.Discussion is made of the g-value relating to the Landau level width and the energy gaps in the density of states under strong magnetic field.     

Temperature dependence of effective mobility edge and inelastic scattering time in silicon MOS inversion layers in strong magnetic fields

Temperature (T) dependence on Hall conductivity (σ_xy) in Si MOS inversion layers measured in 15 T at T=1.4?10 K is investigated by comparing dσ_xy/dN_s, N_s electron concentration, to the calculation based on an effective mobility edge (E_c) model. Temperature dependence of inelastic scattering time is discussed in connection to the T-dependence of E_c.     

Anomalous temperature dependence of paraelectric relaxation at very low temperatures

Hydroxyl doped KCl crystals were subjected to two dimensional stress in a (001)-plane so that the paraelectric hydroxyl dipoles can be described by a double-well system with a small tunneling splitting. In the temperature range from 0.025 to 0.3 K this system exhibits a dielectric relaxation rate that increases with decreasing temperature.     

Wavevector dependence of the two-dimensional plasmon dispersion relationship in the (100) silicon inversion layer

The two-dimensional plasmon dispersion relationship in the inversion layer on (100) p-silicon is studied as a function of wavevector q. This is facilitated by a simple technique which allows plasmon resonances corresponding to several values of q to be excited in the same sample. Over the experimentally obtained values of q the results are in good agreement with theory.     

Plasmon dispersion and intersubband resonance at high wavevectors in Si(100) inversion layers

Deviations from the classical two-dimensional plasmon dispersion are observed at high wavevectors q in electron inversion layers on Si(100) MOS-capacitors with periodically structured gate electrodes. For high inversion electron densities (n_s> 6 × 10¹²cm^?2) an unexpected mass enhancement is extracted from the plasmon dispersion. In addition the plasmon linewidth is found to be significantly larger than predicted from transport experiments. On the same samples non-vertical intersubband resonance transitions are observed with radiation incident normal to the interface. They are excited by an electric field component normal to the surface that is induced by the structured gate electrode.     

Valley splitting and valley degeneracy in n-type silicon (110) inversion layers

The magneto-quantum transport phenomena of silicon (110) n-type inversion layers directly reveal the presence of two types of electrons in i = O and O' electric subband states from their respective quantum oscillations. While the higher i = O' states are occupied by only about 6% of the total electron concentration in our samples, the i = O electrons populate two valley pairs each, the level ladders of which are shifted in energy by less than the Landau splitting in the range where quantum oscillations are observed. We propose this effect to originate from a slight misorientation (Δθ < 0.5°) of the surfaces investigated with respect to the (110) axis. Hence, increasing the gate voltage in a quantizing magnetic field all four i = O valleys are alternatingly occupied in pairs with increasing Landau quantum number in our (110) surfaces contrary to former conclusions on samples with comparable properties. The effect of the magnitude of Δθ on the resulting magneto-quantum oscillations is unimportant for 0 < Δθ ? 0.5° as a result of energy minimization for the total electron system.     

Determination of valley splitting in (100) Si inversion layers

An experimental method is presented to determine the valley splitting in n-channel Si inversion layers. The method is an extension of the “tilted field” experiment under conditions, where the valley splitting is resolved at all tilt angles. In the framework of Ohkawa and Uemura's theory, the bare splitting as well as the effective exchange interaction are obtained directly from the angles of coincidence. The bare valley splitting in the samples investigated is 0.15 meV/10¹² cm^-2 ·n_s. Using experimentally determined parameters we have calculated the oscillatory conductivity σ_xx in a simplified version of Ohkawa and Uemura's model. The calculated curves reproduce the detailed structure of the experimental data for the various tilt angles of the experiment.     

Low temperature saturation of the channel conductivity in silicon inversion layers

The conductivity of silicon inversion layers in MOS structures has been measured in the temperature range 0.03 ? T ? 4.2 K at low carrier densities. The conductivity is activated but saturates at the lowest temperatures for both n- and p-channels. The saturation conductivity is found to increase with reverse substrate bias.     

Surface quantum oscillations in (110) and (111) n-type silicon inversion layers

Shubnikov-de Haas oscillations have been studied for (110) and (111) n-type silicon inversion layers. The measured cyclotron masses m_c = (0.38 ± 0.03)m₀ and m_c = (0.40 ± 0.03)m₀ for (110) and (111) planes, respectively, are larger than theoretically predicted values. The experimental valley degeneracy factor g_v = 2 ± 0.2 for both orientations is also at variance with self consistent calculations. The electronic g-factor depends on the surface carrier concentration and is enhanced over its bulk value. There was no evidence for the occupation of other subbands.     

Observation of valley splitting in (lll) n-type silicon inversion layers

An additional splitting of Shubnikov-de Haas oscillations in (lll) n-type silicon inversion layers in observed. We attribute it to valley splitting, analogous to that previously observed on (100) surface layers, and point out that theoretical explanations of valley splitting, restricted to the valley configuration in (100) inversion layers, are not applicable.     

Structure of silicon oxide on Si(001) grown at low temperatures

Structure of amorphous oxide thin-films grown on Si(001) wafers under an atmosphere of O₃, and also under an atmosphere of afterglow of microwave plasma of O₂ was investigated by X-ray diffraction. Crystallites were found in both oxidation samples as well as in the usual O₂ oxidation [I. Takahashi et al., J. Phys: Condensed Matter 5 (1993) 6525]. Peak intensity and profile in X-ray diffraction responsible for existence and distribution of the crystallites in the oxide film, depend on the oxidation conditions. Least-squares fitting of the data indicates that crystallite growth is less pronounced in O₃ oxidation compared with O₂ oxidation, and the crystallites are located at the interface only in the afterglow of microwave plasma oxidation. Parameters describing the interface morphology indicate that the interface produced by either oxidation procedure is not as smooth as conventional O₂ oxidized samples.     

Studies of electron screening effects on the electron mobility in silicon surface inversion layers

Theoretical calculations of the effects of electron screening of the randomly distributed oxide charges on the electron mobility in surface inversion layers at 4.2 K show that screening substantially enhances the mobility even in weakly inverted surface layer. Surface conductances are measured at 4.2 and 77 K on n-channel MOS transistors with 4 × 10¹¹ ions/cm². Threshold voltages are obtained by matching the inversion layer conductance versus gate voltage data to the theory. The observed mobility magnitude at 4.2 K is about 2000 cm²/V-s which is in excellent agreement with the theory including screening but without adjustable parameters. A conductance tail of several volts wide below the theoretical threshold voltage and a large positive threshold voltage shift are observed at low temperatures which are attributed to an areal inhomogeneous distribution of fast surface states near the silicon conduction band edge.     

Current transport in Pd2Si/n-Si(100) Schottky barrier diodes at low temperatures

The forward current-voltage (I–V) characteristics of Pd₂Si/n-Si(100) Schottky barrier diodes are shown to follow the Thermionic Emission-Diffusion (TED) mechanism in the temperature range of 52-295 K. The evaluation of the experimentalI–V data reveals a decrease of the zero-bias barrier height ( _b0) and an increase of the ideality factor () with decreasing temperature. Further, the changes in _b0 and become quite significant below 148 K. It is demonstrated that the findings cannot be explained on the basis of tunneling, generation-recombination and/or image force lowering. Also, the concepts of flat band barrier height and T ₀-effect fail to account for the temperature dependence of the barrier parameters. The 1n(I _s /T ²) vs 1/T plot exhibits nonlinearity below 185 K with the linear portion corresponding to an activat ion energy of 0.64 eV, a value smaller than the zero-bias barrier height energy (0.735 eV) of Pd₂Si/n-Si Schottky diodes. Similarly, the value of the effective Richardson constant A** turns out to be 1.17 × 10⁴ A m^–2 K^–2 against the theoretical value of 1.12 × 10⁶ A m^–2 K^–2. Finally, it is demonstrated that the observed trends result due to barrier height inhomogeneities prevailing at the interface which, in turn, cause extra current such that theI–V characteristics continue to remain consistent with the TED process even at low temperatures. The inhomogeneities are believed to have a Gaussian distribution with a mean barrier height of 0.80 V and a standard deviation of 0.05 V at zero-bias. Also, the effect of bias is shown to homogenize barrier heights at a slightly higher mean value.