Si Quantum Wells Research Articles

Observation of high mobility 2DHG with very high hole density in the modulation doped strained Ge quantum well at room temperature

Very high two-dimensional hole gas (2DHG) drift mobility of 3100 cm 2 V −1 s −1 is obtained at extremely high density of 4.1×10 12 cm −2 in the modulation doped (MOD), 20 nm thick, strained Ge quantum well (QW) of SiGe heterostructure at room temperature. Obtained 2DHG mobility is higher and the carrier density is about eight times larger than those ever reported for SiGe MOD heterostructures. It is also noted that obtained values are not only the highest ones among 2DHG in the strained Ge QW but also larger than those of two-dimensional electron gas in the strained Si QW.

Cooperative effects in SiGe/Si quantum wells

Studies of the luminescence spectra of Si 1− x Ge x /Si quantum wells with a low Ge content have been carried out in a wide pumping power range. It was shown that a Si 1− x Ge x /Si quantum well spectrum variation observed at low temperatures and high pumping powers is caused by cooperative effects—formation of the electron-hole liquid and multiexciton complexes in quantum wells.

Observation of two-dimensional hole gas with mobility and carrier density exceeding those of two-dimensional electron gas at room temperature in the SiGe heterostructures

Very high two-dimensional hole gas (2DHG) drift mobility of 3100cm2∕Vs is obtained at extremely high density of 41×1011cm−2 in the modulation doped, 20nm thick, strained Ge quantum well (QW) of SiGe heterostructure at room temperature. Very high 2DHG density is achieved by increasing the boron modulation doping, reducing the spacer layer thickness located between it and Ge QW, and increasing the valence-band offset of Ge QW, which also results in the enhancement of mobility. The obtained 2DHG mobility and carrier density exceed those reported for two-dimensional electron gas in the strained Si QW of SiGe heterostructures.

Optical transitions between valley split subbands in biased Si quantum wells

By a tight-binding $s{p}^{3}{d}^{5}{s}^{*}$ model, we study numerically the optical transitions involving the lowest conduction states confined in strained [001] Si quantum wells. These states belong to the fundamental and to the first excited quantum well (QW) subbands, each one split into a doublet by intervalley interaction. Both hard wall and finite SiGe barriers boundary conditions for the QWs are considered. Amplitudes of the doublet splittings as a function of the well width and of a uniform electric field superimposed along the growth direction are first investigated. Then, we study atomic contributions and parity character of the doublet wave functions to derive selection rules for interdoublet optical transitions. Finally, we demonstrate the role of intervalley coupling and the effectiveness of the selection rules here presented, for the interpretation of the absorption spectrum of a $n$-type Si QW between SiGe barriers, evaluated at different temperatures.

Photoluminescence in crystalline silicon quantum wells

Crystalline silicon single quantum wells (QWs) with a minimum Si layer thickness of around 1nm were fabricated by high temperature thermal oxidation of separation by implantation of oxygen (SIMOX) and epitaxial layer transfer (ELTRAN®) silicon-on-insulator (SOI) wafers. Not only Si thickness but also defect-free SOI materials may be important factors for observation of quantum-confined transition in the Si QWs. Si QWs fabricated from SIMOX SOI wafers showed strong interface-mediated luminescence, which is independent of Si thickness, at 1.63 and 1.49eV in the Si thickness range from 1to7nm. On the other hand, Si QWs fabricated from ELTRAN SOI wafers showed very strong thickness dependent luminescence in the range of 1.36–1.77eV without interface-mediated luminescence, where the Si thickness range was 3–1nm. The ability to detect quantum-confined luminescence seems to arise from the use of high-quality defect-free ELTRAN SOI wafers, from suppressed interface state luminescence by high temperature oxidation, and, possibly, from interface matching by crystalline silicon oxide. Hydrogen passivation suggests luminescence from a weak SiO state at around 1.59eV, but luminescence due to quantum confinement is more predominant in the ELTRAN Si QWs.

Conduction intersubband transitions at normal incidence inSi1−xGex quantum well devices

We show theoretically that it is possible to design SiGe-based quantum well structures in whichconduction intersubband transitions are induced by normal incidence infrared radiation. Asp3d5s* tight binding model has been adopted to evaluate the electronic states and opticaltransitions between lowest conduction confined states of a superlattice composed of onepure Ge quantum well separated by SiGe alloys, grown along the [001] direction. We findthat significant optical coupling between confined states in the Ge wells is achieved atnormal incidence radiation by the off-diagonal elements of the mass tensor. The minimumenergy Ge conduction valleys are, in fact, tilted with respect to the [001] growth axis. Forcomparison we show that no such coupling can be realized for the conductionstates confined in a similar structure composed by Si quantum wells because theellipsoids of the lowest conduction valleys are oriented along the growth direction.

Electronic Structure of Si/InAs Composite Channels

AbstractElectronic structure calculations on composite channels, consisting of indium arsenide grown on the technologically relevant (001), (011) and (112)-orientated silicon surfaces are reported. The calculations are performed with NEMO 3-D, where atoms are represented explicitly in the sp3d5s* tight-binding model. The Valence Force Field (VFF) method is employed to minimize the strain. NEMO 3-D enables the calculation of localized states in the quantum well (QW) and their dispersion in the quantum well plane. From this dispersion, the bandgap, its direct or indirect in character, and the associated effective masses of the valence and conduction band can be determined. Such composite bandstructure calculations are demonstrated here for the first time. The numerical results presented here can then be included in empirical device models to estimate device performance. Pure InAs QWs create a direct bandgap material, with a relatively small gap and effective masses of about one order of magnitude smaller than for pure Si QW of equivalent thickness. Si, on the other hand, has a larger bandgap, superior thermal and mechanical properties, and a heavily invested industry. Thus heteroepitaxy of both components is expected to yield a highly optimized overall system. For samples grown along the (001) direction, Si is a direct bandgap material, and deposition of an InAs 3nm layer reduces substantially the hole effective mass and slightly the electronic mass, decreasing the magnitude of the gap. Along the (011) and (112)-growth direction, Si QWs are indirect bandgap material, and deposition of a few InAs layers suffies to make the new material a direct-bandgap heterostructure, decreasing significantly the electronic effective mass. (011) and (112) are the experimentally most relevant growth directions since they prevent heterointerface dipoles.

EnergyDispersion Relations for Holes in Silicon Quantum Wells and Quantum Wires

We calculate the energy dispersion relations in Si quantum wells (QW), E(k2D), and quantum wires (QWR), E(k1D), focusing on the regions with negative effective mass (NEM) in the valence band. The existence of such NEM regions is a necessary condition for the current oscillations in ballistic quasineutral plasma in semiconductor structures. The frequency range of such oscillations can be extended to the terahertz region by scaling down the length of structures. Our analysis shows that silicon is a promising material for prospective NEM-based terahertz wave generators. We also found that comparing to Si QWRs, Si QWs are preferable structures for NEM-based generation in the terahertz range.

Magnetic-field dependence of valley splitting in Si quantum wells grown on tilted SiGe substrates

The valley splitting of the first few Landau levels is calculated as a function of the magnetic field for electrons confined in a strained silicon quantum well grown on a tilted SiGe substrate, using a parameterized tight-binding method. For a zero substrate tilt angle, the valley splitting slightly decreases with increasing magnetic field. In contrast, the valley splitting for a finite substrate tilt angle exhibits a strong and non-monotonous dependence on the magnetic field strength. The valley splitting of the first Landau level shows an exponential increase followed by a slow saturation as the magnetic field strength increases. The valley splitting of the second and third Landau levels shows an oscillatory behavior. The non-monotonous dependence is explained by the phase variation of the Landau level wave function along the washboard-like interface between the tilted quantum well and the buffer material. The phase variation is the direct consequence of the misorientation between the crystal axis and the confinement direction of the quantum well. This result suggests that the magnitude of the valley splitting can be tuned by controlling the Landau-level filling factor through the magnetic field and the doping concentration.

Carrier concentration and mobility profiling in quantum wells by scanning probe microscopy

Charge transport at nanoscale in ultra-scaled semiconductor structures and specifically designed nano-standards is investigated by different scanning probe microscopy (SPM) methods, i.e. scanning capacitance microscopy (SCM) and scanning spreading resistance microscopy (SSRM). A set of dedicated samples, containing B doped Si/Si 0.75Ge 0.25/Si quantum wells (QWs) with different thickness ranging from 20 to 1 nm and with B doping ranging from 1 × 10 18 cm −3 to 5 × 10 18 cm −3 were grown by molecular beam epitaxy. We demonstrated that the combined application of SCM and SSRM, commonly used to measure the local carrier concentration and the resistivity, respectively, allows to determine other physical properties, like the drift mobility in the nanometer thick QWs. Interesting effects were directly observed at nanoscale, like the relevant decrease in hole mobility with the shrinkage of QWs width from 20 to 1 nm, which was explained by the increasing role of surface scattering at the Si/SiGe interface for thinner QWs.

Hole-level structure of double [formula omitted]-doped quantum wells in Si: The influence of the split-off band

We present the electronic structure calculation of two closely p-type δ -doped quantum wells within the lines of the Thomas–Fermi–Dirac (TFD) theory. The distance between the impurity planes as well as the impurity density of the δ -doped wells is varied. The exchange effects are also considered in the present study. We have paid special attention to the split-off band and its influence on the subband hole levels. We also calculate the mobility ratio of double δ -doped (DDD) quantum wells in Si with respect to a single δ -doped (SDD) one, finding the optimum distance between the wells for maximum mobility. Our results are in a good agreement with respect to the experimental data available.

ON THE POSSIBILITY OF AN INTERSUBBAND LASER IN SILICON-ON-INSULATOR

As thin Si quantum wells with oxide barriers have become an experimental reality in silicon-on-insulator technology, we discuss the feasibility of a terahertz laser based on an intersubband transition in such a Si quantum well. Electrons tunnel into an upper two-dimensional subband from a thin polysilicon gate through an ultrathin tunneling oxide and are extracted laterally from the Si well by diffusion. Population inversion arises because lateral diffusion to the contacts can be a faster process than intersubband relaxation and also because the in-plane diffusivity in the upper subband is suppressed by the interaction with a nearby subband characterized by a heavy in-plane mass. Thick oxide layers provide optical confinement and the main optical loss mechanism is the weak free-carrier absorption in thin doped regions in the polysilicon gate and substrate. A voltage on the substrate may provide some tunability of the gain peak.

Energy levels in Si and SrTiO3-based quantum wells with charge image effects

In the present work we develop a theoretical study to analyze how the image charges effects can modify the electronic properties in Si and SrTiO3-based quantum wells. We have used the method based on the calculation of the image charge potential by solving Poisson equation in cylindrical coordinates. The numerical results show that the electron-heavy hole recombination energy can be shifted by more than 200 meV due to the combination of charge image and SiO2 (SrTiO3) interface thickness effects.

Quantum Hall ferromagnetism in a two-valley strained Si quantum well

Tilted field magnetotransport study was performed in a two-valley strained Si quantum well and hysteretic diagonal resistance spikes were observed near the coincidence angles. The spike around filling factor ν = 3 develops into a giant feature when it moves to the high-field edge of the quantum Hall (QH) state and quenches for higher tilt angles. When the spike is most prominent, its peak resistance is temperature independent from T∼20 mK up to 0.3 K, which is different from the critical behavior previously reported near the Curie temperature of the QH ferromagnet in AlAs quantum wells. Our data suggest a strong interplay between spins and valleys near the coincidence.

Study of the interdiffusion effect on the band structures of Si1−xGex∕Si quantum wells

We investigated the influence of thermal interdiffusion on the band structures of Si1−xGex∕Si single quantum wells (SQWs) with a well width of 125Å and Ge concentration x=0.3. Rapid thermal annealing with the temperatures of 750 and 800°C leads to substantial interdiffusion indicated by a significant photoluminescence (PL) blueshift as large as 46meV. The effect is modeled by a Si–Ge atomic interdiffusion at the heterointerface. The band structures and optical transitions of QW after interdiffusion were calculated based on an error function distribution and the 6+2-band k∙p method. The diffusion lengths of the intermixing process are deduced from the PL shift. The thermal dependence of the interdiffusion coefficients follows the Arrhenius law. An activation energy (Ea) for interdiffusion of 2.75eV is obtained. Our investigation indicates that the 6+2-band k∙p formalism is valid for interdiffused Si1−xGex∕Si QWs.

Two sub-band conductivity of Si quantum well

We report on two sub-band/bi-layer transport in double gate SiO 2–Si–SiO 2 quantum well with 14 nm thick Si layer at 270 mK. At symmetric well potential the experimental sub-band spacing changes monotonically from 2.3 to 0.3 meV when the total electron density is adjusted by gate voltages between ∼ 0.7 × 10 16 – 3.0 × 10 16 m - 2 . The conductivity is mapped in large gate bias window and it shows strong non-monotonic features. At symmetric well potential and high density these features are addressed to sub-band wave function delocalization in the quantization direction and to different disorder of the top and bottom interfaces of the Si well. In the gate bias regimes close to second sub-band/bi-layer threshold the non-monotonic behavior is interpreted to arise from scattering from the other electron sub-system with localized or low mobility states.

Subband and transport calculations in double n-type δ-doped quantum wells in Si

The Thomas-Fermi approximation is implemented in two coupled n-type δ-doped quantum wells in Si. An analytical expression for the Hartree-Fock potential is obtained in order to compute the subband level structure. The longitudinal and transverse levels are obtained as a function of the impurity density and the interlayer distance. The exchange-correlation effects are analyzed from an impurity density of 8×1012to6.5×1013cm−2. The transport calculations are based on a formula for the mobility, which allows us to discern the optimum distance between wells for maximum mobility.

Drift mobility in quantum nanostructures by scanning probe microscopy

We used scanning capacitance microscopy (SCM) and scanning spreading resistance microscopy (SSRM) to determine the drift hole mobility in nanometer thick Si∕Si0.75Ge0.25∕Si quantum wells (QWs), from the carrier concentration profiles (obtained by SCM) and the local resistivity values (obtained by SSRM). A relevant decrease of the hole mobility at room temperature was observed when reducing the QW width from 10 nm down to 1 nm. This effect has been explained in terms of the increasing role of surface scattering at the Si∕SiGe interface for lower QW widths.

PHOTOCURRENT ABSORPTION SPECTROSCOPIC STUDY OF Si0.6Ge0.4/Si QUANTUM WELLS

Absorption spectra of Si 0.6 Ge 0.4/ Si quantum wells are characterized by photocurrent measurements. The absorption coefficients of two different transitions, namely the transition between the Si band states and the discrete energy level in quantum wells, and the interlevel transition in quantum wells are deduced. They are directly proportional to (ℏω-ΔE)3/2 and δ(ℏω-Eeh), respectively. The valence band offsets of Si 0.6 Ge 0.4/ Si interface are 297 meV. The ground state energy levels in valence band and conduction band Si 0.6 Ge 0.4/ Si quantum wells are 37 meV and 23 meV, respectively.

Current Induced G-Factor Shift in Modulation Doped Si Quantum Wells

AbstractWe report the observation of a particularly simple effect of spin-orbit coupling which allows for efficient manipulation of spins by an electric current in semiconductor nanostructures. Passing an electric current density of j = 2.5 mA/cm through a modulation doped Si quantum well (density of 5 × 1011 cm-2) perpendicular to an in-plane magnetic field, we observe a shift of the spin resonance of the conduction electrons (CESR) by about 0.1 mT. This shift reverses sign when we invert (i) the current direction, (ii) the magnetic field direction and it vanishes for perpendicular magnetic field. We show that this current-induced shift in g-factor, i.e., its dependence on current and carrier density, its temperature dependence and its anisotropy can be consistently and quantitatively explained in terms of the Bychkov-Rashba coefficient determined earlier from the CESR broadening and the g-factor anisotropy [1]. Other sources of magnetic field (e.g. the Oersted effect) are negligible. This effect can be utilized for g-factor tuning, and thus for local spin manipulation: passing a current through some part of a sample may be utilized to bring those electrons into resonance with a microwave field. These spins are thus excited to Rabi oscillations and, using current pulses of suitable duration, π rotations (or by any other angle) can be achieved.