Silicon Quantum Wells Research Articles

EDESR of impurity centers in nanostructures inserted in silicon microcavities

AbstractWe present the first findings of the new electrically detected electron spin resonance technique (EDESR) which reveal single point defects in the ultra‐narrow silicon quantum wells (Si‐QW) confined by the superconductor δ‐barriers. This technique allows the ESR identification without the application of the external cavity as well as the high frequency (hf) source and recorder, with measuring the only magnetoresistance caused by the hf emission from the δ‐barriers in the presence of the microcavity embedded in the Si‐QW plane. The new resonant positive magnetoresistance data are interpreted here in terms of the interference transition in the diffusive transport of free holes respectively between the weak antilocalization regime in the range of magnetic fields far from the ESR of a paramagnetic point defect located inside or near the conductive channel and the weak localization regime in the range of magnetic fields corresponding to the ESR of that defect. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Quantum Conductance Staircase of Edge Hole Channels in Silicon Quantum Wells

We present the findings for the fractional quantum conductance of holes that is caused by the edge channels in the silicon nanosandwich prepared within frameworks of the Hall geometry. This nanosandwich represents the ultra-narrow p-type silicon quantum well (Si-QW), 2 nm, confined by the {\delta}-barriers heavily doped with boron on the n-type Si (100) surface. The edge channels in the Si-QW plane are revealed by measuring the longitudinal quantum conductance staircase, Gxx, as a function of the voltage applied to the Hall contacts, Vxy, to a maximum of 4e2/h. In addition to the standard plateau, 2e2/h, the variations of the Vxy voltage appear to exhibit the fractional form of the quantum conductance staircase with the plateaus and steps that bring into correlation respectively with the odd and even fractional values.

Investigation of double barrier MOS tunnel diodes with PECVD silicon quantum well

Double barrier metal–oxide–semiconductor tunnel diodes with ultrathin PECVD Si and thermal SiO 2 layers were fabricated. The measured capacitance–voltage and current–voltage characteristics were interpreted and physical parameters of the structures were extracted by means of a theoretical model.

Quantum Supercurrent Transistors in Silicon Quantum Wells Confined by Superconductor Barriers

We present the findings of spin-dependent single-hole and pair-hole transport in plane and across the p-type high mobility silicon quantum wells (Si-QW), 2 nm, confined by the superconductor δ-barriers on the n-type Si (100) surface. The oscillations of the conductance in normal state and the zero-resistance supercurrent in superconductor state as a function of the top gate voltage are found to be correlated by on- and off-resonance tuning the two-dimensional levels of holes in Si-QW with the Fermi energy in the superconductor δ-barriers. The SIMS and STM studies have shown that the δ-barriers heavily doped with boron, 5 × 1021 cm–3, represent really alternating arrays of silicon empty and doped dots, with dimensions restricted to 2 nm. This concentration of boron seems to indicate that each doped dot located between empty dots contains two impurity atoms of boron. The EPR studies show that these boron pairs are the trigonal dipole centres, B+ - B–, that contain the pairs of holes, which result from the negative -U reconstruction of the shallow boron acceptors, 2B0 => B+ - B–. The electrical resistivity, magnetic susceptibility and specific heat measurements demonstrate that the high density of holes in the Si-QW (> 1011 cm–2) gives rise to the high temperature superconductor properties for the δ-barriers. The value of the superconductor energy gap obtained is in a good agreement with the data derived from the oscillations of the conductance in normal state and of the zero-resistance supercurrent in superconductor state as a function of the bias voltage. These oscillations appear to be correlated by on- and off-resonance tuning the two-dimensional subbands of holes with the Fermi energy in the superconductor δ-barriers. Finally, the proximity effect in the S-Si-QW-S structure is revealed by the findings of the quantization of the supercurrent and the multiple Andreev reflection (MAR) observed both across and along the Si-QW plane thereby identifying the spin transistor effect.

EDESR and ODMR of Impurity Centers in Nanostructures Inserted in Silicon Microcavities

We present the first findings of the new electrically- and optically-detected magnetic resonance technique [ED electron spin resonance (EDESR) and (ODMR)] which reveal single point defects in the ultra-narrow silicon quantum wells (Si-QW) confined by the superconductor δ-barriers. This technique allows the ESR identification without the application of the external cavity as well as a high frequency source and recorder, with measuring the only magnetoresistance (EDESR) and transmission (ODMR) spectra within frameworks of the excitonic normal-mode coupling (NMC) caused by the microcavities embedded in the Si-QW plane. The new resonant positive magnetoresistance data are interpreted here in terms of the interference transition in the diffusive transport of free holes respectively between the weak antilocalization regime in the region far from the ESR of a paramagnetic point defect located inside or near the conductive channel and the weak localization regime in the nearest region of the ESR of that defect.

EDESR and ODMR of Impurity Centers in Nanostructures Inserted in Silicon Microcavities

We present the first findings of the new electrically and optically detected magnetic resonance technique [ED electron spin resonance (EDESR) and ODMR], which reveal single point defects in the ultra-narrow silicon quantum wells (Si-QW) confined by the superconductor δ-barriers. This technique allows the ESR identification without application of an external cavity, as well as a high frequency source and recorder, and with measuring only the magnetoresistance (EDESR) and transmission (ODMR) spectra within the frameworks of the excitonic normal-mode coupling caused by the microcavities embedded in the Si-QW plane. The new resonant positive magnetoresistance data are interpreted here in terms of the interference transition in the diffusive transport of free holes, respectively, between the weak antilocalization regime in the region far from the ESR of a paramagnetic point defect located inside or near the conductive channel and the weak localization regime in the nearest region of the ESR of that defect.

Modelling of metal–insulator–semiconductor devices featuring a silicon quantum well

A straight-forward model of a metal–insulator–semiconductor (MIS) device that accommodates an arbitrarily defined insulating potential barrier is described. Using the model, current density–voltage ( J– V) curves of MIS devices featuring a single silicon (Si) quantum well (QW) embedded between silicon dioxide (SiO 2) insulating barriers (Si:QW-MIS devices) are simulated. Furthermore, the electron current density is examined as a function of carrier energy. In the case of a 4 nm QW, a current step is predicted due to rising of the first quasi-bound state of the QW above the conduction band edge of the Si semiconductor substrate. Defects in the SiO 2 barriers are simulated indirectly by varying the electron effective tunnelling mass in SiO 2. Reductions in the SiO 2 electron effective tunnelling mass cause a shift in the position of the current step. An explanation for this shift is provided by taking into account the response of carrier populations at the Si substrate surface to changes in the magnitude of electron tunnelling current.

Stimulated emission of near-infrared radiation by current injection into silicon (100) quantum well

We describe the observation of stimulated emissions by current injections into a silicon quantum well. The device consists of a free standing membrane with a distributed feedback resonant cavity fabricated by state-of-the-art silicon processes. The emission spectra have multimode structures peaked in the near-infrared region above the submilliampere threshold currents at room temperatures. Consequently, electronics and photonics should be able to be converged on chips by using silicon quantum well laser diodes.

Oscillatory spin-polarized tunnelling from silicon quantum wells controlled by electric field

Spin-dependent electronic transport is widely used to probe and manipulate magnetic materials and develop spin-based devices. Spin-polarized tunnelling, successful in ferromagnetic metal junctions, was recently used to inject and detect electron spins in organics and bulk GaAs or Si. Electric field control of spin precession was studied in III-V semiconductors relying on spin-orbit interaction, which makes this approach inefficient for Si, the mainstream semiconductor. Methods to control spin other than through precession are thus desired. Here we demonstrate electrostatic modification of the magnitude of spin polarization in a silicon quantum well, and detection thereof by means of tunnelling to a ferromagnet, producing prominent oscillations of tunnel magnetoresistance of up to 8%. The electric modification of the spin polarization relies on discrete states in the Si with a Zeeman spin splitting, an approach that is also applicable to organic, carbon-based and other materials with weak spin-orbit interaction.

Superconducting properties of silicon nanostructures

We present the findings of the superconductivity in the silicon nanostructures prepared by short time diffusion of boron after preliminary oxidation of the n-type Si (100) surface. These Si-based nanostructures represent the p-type high mobility silicon quantum well (Si-QW) confined by the delta - barriers heavily doped with boron. The ESR studies show that the delta - barriers appear to consist of the trigonal dipole centers, B(+)-B(-), which are caused by the negative-U reconstruction of the shallow boron acceptors, 2B(0)=>B(+)-B(-). The temperature and magnetic field dependencies of the resistance, thermo-emf, specific heat and magnetic susceptibility demonstrate that the high temperature superconductivity observed seems to result from the transfer of the small hole bipolarons through these negative-U dipole centers of boron at the Si-QW - delta - barrier interfaces. The value of the superconductor energy gap obtained is in a good agreement with the data derived from the oscillations of the conductance in normal state and of the zero-resistance supercurrent in superconductor state as a function of the bias voltage. These oscillations appear to be correlated by on- and off-resonance tuning the two-dimensional subbands of holes with the Fermi energy in the superconductor delta - barriers. Finally, the proximity effect in the S- Si-QW -S structure is revealed by the findings of the multiple Andreev reflection (MAR) processes and the quantization of the supercurrent.

Magnetic resonance spectroscopy of single centers in silicon quantum wells

We present the new optically-detected magnetic resonance (ODMR) technique which reveals single point defects in silicon quantum wells embedded in microcavities within frameworks of the excitonic normal-mode coupling (NMC) without the external cavity and the hf source.

Conversion of hole states by acoustic solitons

The hole states in the valence band of a large class of semiconductors are degenerate in the projections of angular momentum. Here we show that the switching of a hole between the states can efficiently be realized by acoustic solitons. The microscopic mechanism of such a state conversion is related to the valence band splitting by local elastic strain. The conversion is studied here for heavy holes localized at shallow and deep acceptors in silicon quantum wells.

Intrinsic optical gain of ultrathin silicon quantum wells from first-principles calculations

Optical gains of ultrathin Si(001) quantum wells are calculated from first principles, and found to be positive because of an intrinsic quantum confinement effect. The gain of the ultrathin silicon film is comparable to that of the bulk GaAs if the carrier density is large enough. The impact of surface structure of the silicon film on the efficiency of light emission is also investigated and we found that ${\text{SiO}}_{2}$ crystal that forms a strainless connection with a Si(001) surface such as quartz enhances optical gain.

Valley degeneracies in (111) silicon quantum wells

(111) silicon quantum wells have been studied extensively, yet no convincing explanation exists for the experimentally observed breaking of sixfold valley degeneracy into two- and fourfold degeneracies. Here, systematic sp3d5s∗ tight-binding and effective mass calculations are presented to show that a typical miscut modulates the energy levels, which leads to breaking of sixfold valley degeneracy into two lower and four raised valleys. An effective mass based valley-projection model is used to determine the directions of valley minima in tight-binding calculations of large supercells. Tight-binding calculations are in better agreement with experiments compared to effective mass calculations.

Phase and amplitude response of the ‘0.7 feature’ caused by holes in silicon one-dimensionalwires and rings

We present findings for the 0.7(2e2/h) feature in the hole quantum conductance staircase that is caused by silicon one-dimensionalchannels prepared by the split-gate method inside the p-type silicon quantum well (SQW) on then-type Si(100) surface. Firstly, the interplay of the spin depolarization with the evolution of the0.7(2e2/h) featurefrom the e2/h to 3/2 e2/h values as a function of the sheet density of holes is revealed by the quantumpoint contact connecting two 2D reservoirs in the p-type SQW. The 1D holesare demonstrated to be spin polarized at low sheet density, because the0.7(2e2/h) feature is closeto the value of 0.5(2e2/h) that indicates the spin degeneracy lifting for the first step of the quantum conductance staircase. The0.7(2e2/h) feature is found to take, however, the value of0.75(2e2/h) when the sheet density increases, thereby giving rise to the spin depolarizationof the 1D holes. Secondly, the amplitude and phase sensitivity of the0.7(2e2/h) feature are studied by varying the value of the external magnetic field and thetop-gate voltage that are applied perpendicularly to the plane of the double-slit ringembedded in the p-type SQW, with the extra quantum point contact inserted in theone of its arms. The Aharonov–Bohm and the Aharonov–Casher conductanceoscillations obtained are evidence of the interplay of the spontaneous spinpolarization and the Rashba spin–orbit interaction (SOI) in the formation of the0.7(2e2/h) feature. Finally,the variations of the 0.7(2e2/h) feature caused by the Rashba SOI are found to take in the fractional form with both theplateaus and steps as a function of the top-gate voltage.

Spin-dependent transport of holes in silicon quantum wells confined by superconductor barriers

We present the findings of spin-dependent single hole and pair-hole transport across the p-type high mobility silicon quantum well (Si-QW) confined by the superconductor delta-barriers on the n-type Si (100) surface. The oscillations of the conductance in normal state and the zero-resistance supercurrent in superconductor state as a function of the bias voltage are found to be correlated by on- and off-resonance tuning the two-dimensional subbands of holes with the Fermi energy in the superconductor barriers.

Spin interference in silicon one-dimensional rings

We present the first findings of the spin transistor effect in the Rashba gate-controlled ring embedded in the p-type self-assembled silicon quantum well that is prepared on the n-type Si (100) surface. The coherence and phase sensitivity of the spin-dependent transport of holes are studied by varying the value of the external magnetic field and the top gate voltage that are applied perpendicularly to the plane of the double-slit ring and revealed by the Aharonov-Bohm (AB) and Aharonov-Casher (AC) conductance oscillations, respectively. Firstly, the amplitude and phase sensitivity of the 0.7.(2e2/h) feature of the hole quantum conductance staircase revealed by the quantum point contact inserted in the one of the arms of the double-slit ring are found to result from the interplay of the spontaneous spin polarization and the Rashba spin-orbit interaction (SOI). Secondly, the values of the AC conductance oscillations caused by the Rashba SOI are found to take the fractional form with both the plateaus and steps as a function of the top gate voltage.

Valley splitting in strained silicon quantum wells modeled with 2° miscuts, step disorder, and alloy disorder

Valley splitting (VS) in strained SiGe∕Si∕SiGe quantum wells grown on (001) and 2° miscut substrates is computed in a magnetic field. Calculations of flat structures significantly overestimate, while calculations of perfectly ordered structures underestimate experimentally observed VS. Step disorder and confinement alloy disorder raise the VS to the experimentally observed levels. Atomistic alloy disorder is identified as the critical physics, which cannot be modeled with analytical effective mass theory. NEMO-3D is used to simulate up to 106 atoms, where strain is computed in the valence-force field and electronic structure in the sp3d5s* model.

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.

Si-Based Resonant Tunneling Devices Using UHV Wafer Bonding

While resonant tunneling-based quantum devices have been studied extensively in compound semiconductors, there has been limited work on silicon-based resonant tunneling diode (RTD) devices, especially with processes that are compatible with scaled silicon CMOS processing. The indirect band gap of Si (and Ge) results in significant challenges for these structures. In the case of RTDs, the biggest problem has been the ability to fabricate high quality dielectric - semiconductor interfaces, and high quality crystalline silicon quantum wells, at the dimensions required for resonant tunneling devices. To date, most approaches used to demonstrate siliconbased RTDs have been based on using single crystal, epitaxial dielectrics, with lattice constants similar to those of silicon, in order to allow for the growth of high quality, single crystal silicon for the quantum well. Past attempts to produce silicon-based resonant tunneling devices based on CaF2 barriers have been hampered by poor quality interfaces and the inability to grow high quality silicon on single crystal CaF2.