C. J. Emeleus

Scattering mechanisms affecting hole transport in remote‐doped Si/SiGe heterostructures

Boron modulation‐doped Si/SiGe heterojunctions have been grown by molecular beam epitaxy. The two‐dimensional hole gas formed along the heterojunction, just inside the alloy, has a sheet density in the range 2–5×1011 cm−2 and a typical mobility at 5 K of 2000 cm2 V−1 s−1. An explanation for the magnitude of the mobility is sought by considering likely scattering mechanisms, namely those due to remote impurities, interface roughness, alloy disorder, and interface impurities. A self‐consistent model is used to determine the sheet density in terms of structural and energy parameters and dopant concentrations in the heterostructure. It is shown that the presence of negatively charged impurities at the heterojunction provides the basis for a consistent interpretation of the experimental results.

Low temperature characterization of modulation doped SiGe grown on bonded silicon‐on‐insulator

Modulation doped pseudomorphic Si0.87Ge0.13 strained quantum wells were grown on bonded silicon‐on‐insulator (SOI) substrates. Comparison with similar structures grown on bulk Si(100) wafers shows that the SOI material has higher mobility at low temperatures with a maximum value of 16 810 cm 2/V s for 2.05×1011 cm−2 carries at 298 mK. Effective masses obtained from the temperature dependence of Shubnikov–de Haas oscillations have a value of (0.27±0.02) m0 compared to (0.23±0.02) m0 for quantum wells on Si(100) while the cyclotron resonance effective masses obtained at higher magnetic fields without consideration for nonparabolicity effects have values between 0.25 and 0.29 m0. Ratios of the transport and quantum lifetimes, τ/τq=2.13±0.10, were obtained for the SOI material that are, we believe, the highest reported for any pseudomorphic SiGe modulation doped structure and demonstrates that there is less interface roughness or charge scattering in the SOI material than in metal–oxide–semiconductor field ef...

Study of Hall and effective mobilities in pseudomorphic Si1−xGex p-channel metal–oxide–semiconductor field-effect transistors at room temperature and 4.2 K

A study of several Si0.8Ge0.2 p-channel heterostructures with self-aligned poly-Si metal–oxide–semiconductor gates were carried out. A novel fabrication process was developed which is compatible with the strained Si/SiGe system, and it has allowed Hall and resistivity measurements to be performed at room temperature and at 4.2 K. The structures were numerically modelled to calculate the charge distribution with temperature and with gate voltage and the results have shown good agreement with experiment. Hall measurements at 4.2 K have shown consistent SiGe channel Hall mobility enhancements of ×3 over the SiO2/Si channels in the same devices. Room temperature effective mobilities were measured for a buried Si0.8Ge0.2 p-channel metal–oxide–semiconductor field-effect transistor heterostructure using capacitance–voltage measurements to calculate the carrier density. Mobilities are consistently over 300 cm2/V s and the low temperature studies, together with measurements of comparable modulation doped heterostr...

Growth studies on Si0.8Ge0.2 channel two-dimensional hole gases

We report a study of the influences of MBE conditions on the low‐temperature mobilities of Si/Si0.8Ge0.2 2DHG structures. A significant dependence of 2DHG mobility on growth temperature is observed with the maximum mobility of 3640 cm2 V−1 s−1 at 5.4 K being achieved at the relatively high‐growth temperature of 640 °C. This dependence is associated with a reduction in interface charge density. Studies on lower mobility samples show that Cu contamination can be reduced both by growth interruptions and by modifications to the Ge source; this reduction produces improvements in the low‐temperature mobilities. We suggest that interface charge deriving from residual metal contamination is currently limiting the 4‐K mobility.

Back gating of a two-dimensional hole gas in a SiGe quantum well

A device comprising a low-resistivity, n-type, Si substrate as a back gate to a p-type (boron), remote-doped, SiGe quantum well has been fabricated and characterized. Reverse and forward voltage biasing of the gate with respect to the two-dimensional hole gas in the quantum well allows the density of holes to be varied from 8 × 1011 cm–2 down to a measurement-limited value of 4 × 1011 cm–2. This device is used to demonstrate the evolution with decreasing carrier density of a re-entrant insulator state between the integer quantum Hall effect states with filling factors 1 and 3.

Hole transport in Si0.8Ge0.2 quantum wells at low temperatures

The temperature dependence of the resistivity and the Hall coefficient of two-dimensional hole gases in the range 0.3-20 K has been investigated experimentally. The samples used in this study, grown by MBE, were Si/Si 0.8 Ge 0.2 single heterojunctions, modulation doped with boron. Carrier mobilities at 5 K are in the range 2000-3750 cm 2 V -1 s -1 , depending on growth conditions. From the results, which suggest the presence of weak localisation and hole-hole interaction effects as well as a temperature dependence of the Boltzmann conductivity, it is deduced that charged interface states are the most important scattering centres, in agreement with previous work.

Peculiarities in the electron properties of δ〈Sb〉-layers in epitaxial silicon. III. Electron–phonon relaxation

Complex studies of weak electron localization, electron–electron interaction, and electron overheating in Si crystals containing a δ〈Sb〉-layer with various concentrations of Sb atoms are carried out in order to obtain information on the characteristic times of inelastic electron relaxation. The temperature dependence of the electron–phonon relaxation time τep derived from the electron overheating effect can be described by the dependence τep∝T−p, where p≅3.7±0.3, which corresponds to the case qTl<1 (qT is the wave vector of the thermal phonon and l the electron mean free path).

Peculiarities of electronic properties of δ〈Sb〉 layers in epitaxial silicon. IV. Hopping conductivity and nonlinear effects

The temperature dependence of the kinetic electronic characteristics (conductivity, magnetoresistance, Hall e.m.f.) is studied in the temperature interval 3–50 K on epitaxial silicon crystals having a δ 〈Sb〉 layer with sheet concentrations of Sb atoms 1×1013 and 5×1012 cm−2. The shape of the current–voltage characteristics is determined at various temperatures. It is found that the low-temperature kinetic phenomena in these objects are governed by the hopping mechanism of conductivity. A variable range hopping conductivity is observed at sufficiently low temperatures (<10 K). The nonlinearity of the current–voltage characteristics is explained by the theory of non-Ohmic hopping conductivity in moderately strong electric fields.

Weak localisation and interaction of electrons in MBE δ -layers inSi

Temperature (1.6–20K) and magnetic field (up to 2.5T) dependences of quantum corrections to the conductivity and Hall coefficient ofSi epitaxial films containing a δ-layer of different sheet densityNSb are studied. The quantum corrections are due to the effects of electron weak localization (WL) and electron-electron interaction (EEI). Analysis of the quantum corrections made it possible to determine the temperature dependence of electron phase relaxation time τϕ, the spin-orbit interaction time τ80 and the screening factorF. It is found that the dependence τϕ(T) is determined by the electron-electron scattering processes, τϕ∝T−p withp≈1, and an increase in the parameterF with decreasing electron concentrationn in the δ-layer is related to the specific features of the screening processes in two-dimensional electron systems.