Si Quantum Wells Research Articles

On the cross-over between 2-dimensional and 3-dimensional growth in Si/Ge n/Si quantum wells

We report Raman scattering and low temperature photoluminescence measurements performed on a series of Molecular-Beam-Epitaxy-grown Si/Ge n /Si quantum wells with n varying from 3 to 6. Our results are consistent with a gradual evolution of the Si Ge interface which starts with Ge segregation and formation of terraces for low Ge coverage, to smooth 2-dimensional Ge layers, bounded by interfacial alloy layers, for coverages superior to 4-monolayers.

Theory of interface roughness scattering in quantum wells

Interface roughness strongly influences the transport properties of thin quantum wells. The commonly used model of interface roughness scattering approximates the well potential by infinitely high barriers. By using SiGe as an example, we demonstrate that this approximation strongly overestimates interface roughness scattering for narrow quantum wells. Our calculations show that the low-temperature mobilities of two-dimensional hole gases in SiGe and of electrons in strained Si quantum wells are limited by interface roughness scattering for well widths below 20-50 Å. The discussed effect of finite barrier heights on carrier scattering should be considered in transport investigations of materials such as GaAs/AlGaAs and Si/SiGe, when the well width is in the range of 100 Å or below.

Mechanisms and device applications of light emitting phenomena of Si/Si1−xGex/Si quantum wells

Photoluminescence (PL) intensity from SiGe mixed crystals was investigated as a function of the growth temperature and/or atomic-hydrogen irradiation rate during molecular beam epitaxy (MBE). As a result, a clear one-to-one correspondence between PL intensity and the surface segregation length of Ge was obtained. This indicated that formation of Ge–Ge pairs during MBE is a key factor in determining PL intensity. Theoretical calculations showed that localized Si–Ge bonds cause s-like symmetry in the conduction band bottom and p-like symmetry in the valence band top. However, pairing of Ge–Ge atoms changes the local symmetry of the Si–Ge bonds and destroys the s-like symmetry in the conduction band. Such calculations explain the experimental results. This new knowledge triggered the development of “Ge-segregation-controlled MBE,” which successfully enhanced the PL intensity. Consequently, a preliminary optoelectronic device, operating at 77 K with a light emitting diode and detector on the same wafer, was fabricated.

Quantum confined Stark shifts in type-I SiGe quantum structures

Recently observed photoluminescence peak shifts toward higher energy in a longitudinal electric field in type-I Si 1− x Ge x Si quantum wells and superlattices were attributed to a relatively large decrease of the exciton binding energy compared with quantum-confined Stark red shifts. However, careful investigation of the calculated and experimental results show that the observed blue shift can not be fully explained only with the reduction of exciton binding energy. The variational calculation adopted in the previous studies could not be justified for the moderate field strength regime, especially in the shallow quantum well. We suggest that the amounts of the shift calculated based on an Airy function approach can give a suitable account of the observed blue shift.

Progress toward silicon-based intersubband lasers

Design results are presented for the quantum parallel laser (QPL) at 1–20 μm wavelengths and the cryogenic 4–20 μm quantum cascade laser (QCL). For 1–2 μm lasing, the optimum multiple quantum well heterostructures are Si quantum wells (QWs) confined by wide-gap lattice-matched semiconductor layers, especially the Si/ZnS, Si/BeSeTe, Si/γ–Al2O3, Si/CeO2, and Si/SiOx systems (SiOx is a crystalline suboxide). The electrically pumped 300 K unipolar p-i-p. QPL consists of tightly coupled QWs exhibiting coherent transport of carriers on superlattice (SL) minibands. A good QPL candidate is the symmetrically strained Gen–Sin SL grown on relaxed Si0.5Ge0.5. Local-in-k population inversion is engineered between two valence minibands. Our calculations indicate that the p-i-p QCL is feasible in Ge–Si or in lattice-matched Si0.63Ge0.33C0.04/Si. The oscillator strength fz=0.1 calculated for the 8 ML×8 ML Si/ZnS zone-folded SL is insufficient for 1.1 μm band-to-band lasing; however, the in-plane dispersion of Si QWs in Si/ZnS SLs shows valence subbands that are sufficiently nonparabolic for local-in-k lasing in QPLs and QCLs.

Investigation of Ge on Si(100) quantum wells by photoelectron spectroscopies

In the present work we report on the evolution of the electronic states as a function of Ge overlayer thickness ranging from a fraction of a monolayer to few tens of monolayers. The Ge states above the Si valence band top could be clearly detected at every overlayer thickness d. For d<∼6 Å, the Ge state density exhibits a double linear edge. Upon increasing the overlayer thickness the split between the two edges decreases and the double edge structure disappears for d>∼6 Å, i.e., at the critical thickness for the transition from strained to unstrained overlayers. For these larger thicknesses, the Ge edge becomes broad and featureless. The energy separation between the Ge onset and the Si valence top increases up to 0.70 eV at larger overlayer thicknesses.

Thomas-Fermi approximation inp-type δ-doped quantum wells of GaAs and Si

Thomas-Fermi calculations of the hole subband structure in p-type d-doped Si and GaAs quantum wells are carried out for different values of impurity concentration. Results are compared with previous self-consistent calculations and with some experimental reports, and very good agreement is found. In particular, the result of hole ground level from this model is exactly equal to the value reported for the experimental system with the smallest impurity spreading that has been achieved. @S0163-1829~98!04507-X#

GROUND STATE ENERGY LEVEL OF Si-BASE QUANTUM WELLS DETECTED BY ADMITTANCE SPECTROSCOPY

The admittance spectroscopy technique has been used to study the hole confinement in Si/Si1-xGex/Si and boron highly doped superthin Si quantum wells. Based on the carrier thermal emission medel of the carriers in the well, the value of activation energy derived from the admittance spectra was considered as the distance from the position of the heavy hole ground state in the well to the top of it . For Si/Si1-xGex/Si quantum wells we found the value of activation energy increased with increasing annealing time at 800℃. The phenomena could be explained the annealing-induced interdiffusion of Si and Ge, the interface broadening, the weakening of quantum confinement, the decrease of the position of the heavy hole ground state, and so the increase of the activation energy. At the annealing temperature of 900℃, we only see the decrease of the value of activation energy with increasing annealing time. For boron highly doped superthin Si quantum wells due to quantum confinement effect, we observed the activation energy are different for samples with the same doping density but different well widths.

Feasibility Of Novel Si-Based Interminiband Lasers

AbstractWe have designed a parallel interminiband lasing in superlattice structures of coherently strained Si0.5Ge0.5/Si quantum wells (QWs). Population inversion is achieved between the non-parabolic heavy-hole valence minibands locally in-k-space. Lasing transition is at 5.4μm. Our analysis indicates that an optical gain of 134/cm can be obtained when the laser structure is pumped with a current density of 5kA/cm2.

Photoluminescence fromSi/SiO2single quantum wells by selective excitation

We have studied the photoluminescence (PL) mechanism of crystalline Si single quantum wells sandwiched between ${\mathrm{SiO}}_{2}$ layers by site-selective excitation spectroscopy. The asymmetric PL spectrum is observed in the red spectral region at low temperatures and can be divided into two PL bands. The peak energy of the strong PL band is almost independent of the well thickness and appears near $\ensuremath{\sim}1.65\mathrm{eV}.$ Under selective excitation at energies within the weak PL band, TO-phonon structures are observed in both PL and PL polarization spectra. Steplike TO-phonon structure in the PL spectrum of two-dimensional Si quantum wells is similar to that of zero-dimensional Si nanocrystals. The PL properties of very thin Si quantum wells are discussed.

Interface Morphology of Gen / Si Quantum Wells Studied with Raman Spectroscopy and High Resolution X-Ray Diffraction

We report Raman scattering and high-resolution X-ray diffraction measurements performed on a series of MBE-grown Si / Gen / Si quantum wells with n varying from 3 to 6. Our results are consistent with a gradual evolution of the Si / Ge interface which starts with Ge segregation and formation of terraces for low Ge coverage, to smooth 2D Ge layers, bounded by interfacial alloy layers, for coverages superior to 4M Ls.

Resonant Third-Order Optical Susceptibility ?(3)(3?) in Asymmetric Coupled Si1?xGex/Si Quantum Wells

We have calculated the enhancement of the third-order nonlinear optical susceptibility owing to the multi-photon process associated with the intersubband transition in the valence band in asymmetric coupled Si1—xGex/Si quantum wells. The hole states are described by the 6×6 Luttinger-Kohn Hamiltonian which incorporates the heavy-hole, light-hole and spin–orbit split-off bands and the effect of strain of lattice constants in the multi-quantum-well structure has been taken into account. In order to optimize the resonant third-order susceptibility the layer thicknesses of wells and barriers are selected in such a way that the energy spacings between the bound states in the valence band are nearly equal and the product of the dipole matrix elements of the relevant intersubband transitions is maximized. The step structure in the barrier height has been also considered. We found that the susceptibility χ(3)(3ω) is one or two orders of magnitude larger than that of the bulk Si.

Operation of a novel negative differential conductance transistor fabricated in a strained Si quantum well

The operation of a negative differential conductance (NDC) transistor fabricated on a high-mobility Si/Si/sub 1-x/Ge/sub x/ heterostructure wafer is described. The drain characteristic of this device shows a large NDC with current peak-to-valley ratios as high as 600 (100) at T=0.4 K (T=1.3 K). The NDC can be modulated over a wide range of current levels by either of two separately-contacted gate electrodes. The device shows bistable switching behaviour in both current- and voltage-controlled circuit configurations. The novel operating principle of this transistor is described, along with its potential for future logic and memory applications.

Visible luminescence from silicon quantum dots and wells

We discuss the mechanism of efficient photoluminescence (PL) from Si quantum dots and wells. Luminescence properties of SiO 2-capped Si nanocrystals are different from those of H-terminated Si nanocrystals, but are similar to those of two-dimensional Si quantum wells sandwiched by SiO 2 layers. The size dependence of PL properties and resonantly excited PL spectra of SiO 2-capped Si nanocrystals indicate that excitons are localized near the nanocrystal surface and the strong coupling of localized excitons to surface oxide vibrations causes broad PL spectra in the visible spectral region. The localization of excitons near the interface between crystalline Si (c-Si) and surface oxide layer is attributed to efficient luminescence in nanoscale c-Si/SiO 2 systems such as zero-dimensional nanocrystals and two-dimensional quantum wells

Negative differential conductance in lateral double-barrier transistors fabricated in strained Si quantum wells

The operation of a laterally patterned negative differential conductance (NDC) device based on hot-electron effects in strained Si quantum wells is demonstrated. The device consists of a small, etch-defined dot region connected to narrow leads on either side via point-contact tunnel barriers. The entire device is fabricated on a high-mobility Si/Si1−xGex heterostructure wafer using electron-beam lithography and low-damage reactive-ion etching. At T=0.4 K (T=1.3 K), the drain characteristic of this device shows a pronounced NDC region with a peak-to-valley ratio (PVR) greater than 600 (100). The PVR is reduced with increasing temperature, with remnants of the NDC observable to 30 K. The NDC is attributed to phonon emission by hot electrons injected into the dot region.

Controlled growth of SiO2 tunnel barrier and crystalline Si quantum wells for Si resonant tunneling diodes

Two methods for producing Si-oxide barriers upon which crystalline Si layers can be grown are presented. One method entails oxide island nucleation on a clean vicinal Si(001) surface. The second method makes use of void formation in ultrathin oxides on the Si(100) surface at elevated temperatures. Either method results in an oxide barrier which is porous and the exposed Si within these pores can serve as a way to seed c-Si overgrowth. We demonstrate that it is feasible to grow crystalline Si overlayers on top of such porous oxide barriers, while on the continuous Si-oxide surface, only amorphous or nanocrystalline Si layer overgrowth can be achieved. The controlled oxide growth and Si overgrowth on the oxide can find possible applications in Si-based resonant tunneling devices, optoelectronics, and other Si-based nanoelectronics.

Fabrication and band alignment of pseudomorphic Si1 − C , Si1 − − Ge C and coupled quantum well structures on Si substrates

The structural and photoluminescence (PL) properties of several types of pseudomorphic Si 1-x-y Ge x C y quantum well (QW) structures grown by solid-source molecular beam epitaxy on (0 0 1) Si substrates are described. Optimum Si 1-y C y growth takes place at a substrate temperature of about 550°C and a growth rate ≤ 1 A/s. Well-defined alloy layers with no defects or SiC precipitates are observed by transmission electron microscopy (TEM). Excitonic band edge related PL is emitted from Si 1-y C y /Si multiple QWs (MWQ). The band gap of strained Si 1-y C y is drastically reduced by about ΔE = - y x 6.5 eV. Reducing the width of Si 0.99 C 0.01 layers results in a PL blueshift up to 45 meV which is attributed to the strong (weak) quantum well confinement of Δ(2) valley electron (light hole) states. The band alignment in Si 1-y C y /Si QWs is basically explained by the strain-induced shift of levels due to C incorporation. In Si 1-x-y Ge x C y QWs, compressive strain caused by Ge is partially compensated by C and the band gap increases by ΔE = y x 2.4 eV. Si 1-y C y , as well as Si 1-x-y Ge x C y QWs give rise to spatially direct PL transitions. Closely spaced Si 1-y C y /Si 1-x Ge x double quantum wells (DQW) give rise to spatially indirect PL recombination of Δ(2) electrons confined in the Si 1-y C y layers and heavy holes localized in the Si 1-x Ge x layers. The no-phonon transitions and the integrated PL intensity from thin DQWs are strongly enhanced compared to SQWs. In Hall transport studies, Si 1-y C y and SiGeC alloys on Si reveal electron and hole mobilities which are well comparable to Si and SiGe or even improved. C alloying provides a significant extension of the possibilities in band structure engineering of Group-IV semiconductors.

Determination of spin- and valley-split energy levels in strained Si quantum wells

The energy spacing of spin- and valley-split energy levels in strained Si quantum wells is determined using tilted-magnetic-field Shubnikov - de Haas measurements. The effective Landé g-factor, , is determined both as a function of electron sheet concentration, , and Landau level occupation number, . The value of is determined from the angle at which the longitudinal magnetoresistance minima of adjacent spin-split Landau levels are equal. For the coincidence of the and levels, increases from to as is decreased from to at T = 0.4 K. In the same sample, with fixed electron concentration of , for , and oscillates between average values of and , for lower values of . The magnitude of the valley splittings has also been investigated; for a perpendicular magnetic field of , a splitting of is determined, which is several times smaller than values for Si inversion layers reported in the literature.

Si:SiGe quantum wells grown on (118) substrates: Surface morphology and transport properties

We have grown strained Si quantum wells on relaxed Si0.7Ge0.3 buffer layers using vicinal (118) silicon substrates. Compared to conventional (001) substrates the surface is tilted by 10° towards the [110] direction resulting in terraces with step edges which run parallel to [11̄0]. The surface morphology of the layers shows “cross-hatching” characteristic of relaxed SiGe films grown on Si substrates. However, the cross-hatching is not orthogonal but aligns along directions in which (111) planes intersect the (118) surface. We have measured the low temperature transport properties of the two-dimensional electron gas confined within the strained Si channel. When measured with current flowing parallel to the step edges the electron mobility is approximately four times larger than that measured in a perpendicular direction showing the strong elastic scattering associated with the step edges. In contrast the single particle relaxation time is almost identical for the two different orientations.

Many-body carrier interaction effects in quantum wells

Many-body carrier interaction effects shift the ground- and excited-state subband energy levels of quantum wells. We derive carrier-carrier Coulomb exchange and direct interaction energies as a function of carrier density, quantum well composition, and well width. For values of these parameters of physical interest, the excited-state exchange energy is small compared to the ground-state exchange energy. Our expressions for the ground- and excited-state direct interactions yield considerably smaller energy values than those reported by previous investigators. This is due to the fact that in our derivations we differentiate between quantum well width, carrier doping width, and ground- and excited-state wave function effective confinement lengths. Direct interaction energies increase linearly with carrier density, and for large carrier densities substantially negate exchange interaction effects. We illustrate carrier-carrier interaction effects for the heavy-hole subband of electrically neutral, p-type, isolated strained-layer ${\mathrm{Ge}}_{\mathrm{x}}$ ${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$ /Si quantum wells.