Spin-orbit-coupling effects on the valence-band structure of strained semiconductor quantum wells

Topologically-Enhanced Thermoelectric Properties in Bi2Te3-Based Compounds: Effects of Grain Size and Misorientation

Topological insulators (TIs) and thermoelectric (TE) materials seem to belong to distinct physical realms; however, in practice, they both share common characteristics. Introducing concepts from TIs into TE materials to enhance their performance and achieve better understanding of electronic transport requires extensive research. Particularly, grain size, misorientation, and grain boundary (GB) character are of utmost importance to attain effective charge carrier transport in TE polycrystals; these factors, however, have not been thoroughly explored. Herein, we investigate the correlation between grain size, misorientation, and lattice strain in Bi2Te3 and its TI signature, aiming to improve its TE performance. We reveal an unusual behavior showing that electron mobility increases upon the increase of grain size, reaching at a maximum value of 495 cm2/V·s for an optimum grain size of 600 nm and most-frequent GB misorientation angle of 60° and then decreases with increasing grain size. It is also indicated that the combined effects of grain size reduction and point defects induce lattice strain in the Bi2Te3-matrix that is essential to trigger the TI contribution to TE transport. This trend is corroborated by first-principles calculations showing that compressive strains form multiple valleys in the valence band and opens the TI band gap. Such a combination of physical phenomena in a well-known TE material is unique and can promote our understanding of the nature of TE transport with implications for TE energy conversion.

Suppression of shot noise in a spin-orbit coupled quantum dot

We study theoretically the transport properties of electrons in a quantum dot system with spin-orbit coupling. By using the quantum master equation approach, the shot noise and skewness of the transport electrons are calculated. We obtain super-Poisson noise behaviour by investigating the full counting statistics of the transport system. We discover super-Poisson behaviour is more obvious with the spin polarization increasing. More importantly, we discover the suppression of shot noise induced by spin-orbit coupling. The value of shot noise is gradually decreasing when spin-orbit coupling strength increases.

Easy method to measure the packaging-induced stress of a semiconductor laser diode by lasing wavelength shifting

In this paper, the packaging-induced stresses are theoretically calculated by modeling multilayered structures for different packaging structures. We report a method to measure the packaging-induced stress of a laser diode array (LDA) by comparing the emission wavelength of the single emitter located in the middle of a laser bar before and after packaging. The wavelength is tested under a low duty cycle (50 μs/10 Hz, DC 0.05%) to eliminate the thermal effect to wavelength shifting. Experimental calculation results for the packaging-induced stress of LDAs are in good agreement with the theoretical calculations and simulation results. For a GaAs laser bar, we find the packaging stresses are compression stresses, which make the emission wavelength blue-shift in terms of 1.09×10^-2  nm/MPa. We propose a mapping of packaging-induced stress distribution in laser bars on a microscopic scale by considering the emission spectra of each emitter in a laser bar. Compared to single-emitter resolved photo-current or micro-photoluminescence measurements, as proposed by other authors, we offer a much easier tool to test and map the distribution of packaging-induced stress in laser bars.

Recent advances in Sb-based III-V nanowires

Owing to the high mobility, narrow bandgap, strong spin-orbit coupling and large g-factor, Sb-based III-V nanowires (NWs) attracted significant interests in high speed electronics, long-wavelength photodetectors and quantum superconductivity in the past decade. In this review, we aim to give an integrated summarization about the recent advances in binary as well as ternary Sb-based III-V NWs, starting from the fundamental properties, NWs growth mechanism, typical synthetic methods to their applications in transistors, photodetectors, and Majorana fermions detection. Up to now, famous NWs growth techniques of solid-source chemical vapor deposition (CVD), molecular beam epitaxy, metal organic vapor phase epitaxy and metal organic CVD etc have been adopted and developed for the controllable growth of Sb-based III-V NWs. Several parameters including heating temperature, III/V ratio of source materials, growth temperature, catalyst size and kinds, and growth substrate play important roles on the morphology, position, diameter distribution, growth orientation and crystal phase of Sb-based III-V NWs. Furthermore, we discuss the photoelectrical applications of Sb-based III-V NWs such as field-effect-transistors, tunnel diode, low-power inverter, and infrared detectors etc. Importantly, due to the strongest spin-orbit interaction and giant g-factor among all III-V semiconductors, InSb with the geometry of one-dimension NW is considered as the most promising candidate for the detection of Majorana fermions. In the end, we also summarize the main challenges remaining in the field and put forward some suggestions for the future development of Sb-based III-V NWs.

Cuprous halides semiconductors as a new means for highly efficient light-emitting diodes

In group-III nitrides in use for white light-emitting diodes (LEDs), optical gain, measure of luminous efficiency, is very low owing to the built-in electrostatic fields, low exciton binding energy, and high-density misfit dislocations due to lattice-mismatched substrates. Cuprous halides I-VII semiconductors, on the other hand, have negligible built-in field, large exciton binding energies and close lattice matched to silicon substrates. Recent experimental studies have shown that the luminescence of I-VII CuCl grown on Si is three orders larger than that of GaN at room temperature. Here we report yet unexplored potential of cuprous halides systems by investigating the optical gain of CuCl/CuI quantum wells. It is found that the optical gain and the luminescence are much larger than that of group III-nitrides due to large exciton binding energy and vanishing electrostatic fields. We expect that these findings will open up the way toward highly efficient cuprous halides based LEDs compatible to Si technology.

Electrostatic actuated strain engineering in monolithically integrated VLS grown silicon nanowires

In this paper we demonstrate the fabrication and application of an electrostatic actuated tensile straining test (EATEST) device enabling strain engineering in individual suspended nanowires (NWs). Contrary to previously reported approaches, this special setup guarantees the application of pure uniaxial tensile strain with no shear component of the stress while e.g. simultaneously measuring the resistance change of the NW. To demonstrate the potential of this approach we investigated the piezoresistivity of about 3 μm long and 100 nm thick SiNWs but in the same way one can think about the application of such a device on other geometries, other materials beyond Si as well as the use of other characterization techniques beyond electrical measurements. Therefore single-crystal SiNWs were monolithically integrated in a comb drive actuated MEMS device based on a silicon-on-insulator (SOI) wafer using the vapor-liquid-solid (VLS) growth technique. Strain values were verified by a precise measurement of the NW elongation with scanning electron microscopy (SEM). Further we employed confocal μ-Raman microscopy for in situ, high spatial resolution measurements of the strain in individual SiNWs during electrical characterization. A giant piezoresistive effect was observed, resulting in a fivefold increase in conductivity for 3% uniaxially strained SiNWs. As the EATEST approach can be easily integrated into an existing Si technology platform this architecture may pave the way toward a new generation of nonconventional devices by leveraging the strain degree of freedom.

Strain-induced enhancement of plasma dispersion effect and free-carrier absorption in SiGe optical modulators

The plasma dispersion effect and free-carrier absorption are widely used to change refractive index and absorption coefficient in Si-based optical modulators. However, the weak free-carrier effects in Si cause low modulation efficiency, resulting in large device footprint and power consumption. Here, we theoretically and experimentally investigate the enhancement of the free-carrier effects by strain-induced mass modulation in silicon-germanium (SiGe). The application of compressive strain to SiGe reduces the conductivity effective mass of holes, resulting in the enhanced free-carrier effects. Thus, the strained SiGe-based optical modulator exhibits more than twice modulation efficiency as large as that of the Si modulator. To the best of our knowledge, this is the first demonstration of the enhanced free-carrier effects in strained SiGe at the near-infrared telecommunication wavelength. The strain-induced enhancement technology for the free-carrier effects is expected to boost modulation efficiency of the most Si-based optical modulators thanks to high complementary metal-oxide-semiconductor (CMOS) compatibility.

Spin photocurrent spectra induced by Rashba- and Dresselhaus-type circular photogalvanic effect at inter-band excitation in InGaAs/GaAs/AlGaAs step quantum wells

: Spin photocurrent spectra induced by Rashba- and Dresselhaus-type circular photogalvanic effect (CPGE) at inter-band excitation have been experimentally investigated in InGaAs/GaAs/AlGaAs step quantum wells (QWs) at room temperature. The Rashba- and Dresselhaus-induced CPGE spectra are quite similar with each other during the spectral region corresponding to the transition of the excitonic state 1H1E (the first valence subband of heavy hole to the first conduction subband of electrons). The ratio of Rashba- and Dresselhaus-induced CPGE current for the transition 1H1E is estimated to be 8.8±0.1, much larger than that obtained in symmetric QWs (4.95). Compared to symmetric QWs, the reduced well width enhances the Dresselhaus-type spin splitting, but the Rashba-type spin splitting increases more rapidly in the step QWs. Since the degree of the segregation effect of indium atoms and the intensity of build-in field in the step QWs are comparable to those in symmetric QWs, as proved by reflectance difference and photoreflectance spectra, respectively, the larger Rashba-type spin splitting is mainly induced by the additional interface introduced by step structures.

Bound-to-continuum absorption with tunneling in type-II nanostructures: a multiband source-radiation approach

We convert calculations of the bound-to-continuum absorption in type-II semiconductor quantum wells into an equivalent source-radiation problem under the effective-mass approximation with band mixing. Perfectly matched layers corresponding to the eight-band Luttinger-Kohn Hamiltonian are introduced to incorporate the effect of quasi-bound states in open regions. In this way, the interplay between quantum tunneling and optical transitions is fully taken into account. From resulted lineshapes of the Fano resonance, we can evaluate tunneling rates of these metastable states and related absorption strengths relative to those of the continuum. The approach here is useful in estimations of carrier extraction rates from type-II nanostructures for photovoltaic applications.

Observation of strong anisotropic forbidden transitions in (001) InGaAs/GaAs single-quantum well by reflectance-difference spectroscopy and its behavior under uniaxial strain

The strong anisotropic forbidden transition has been observed in a series of InGaAs/GaAs single-quantum well with well width ranging between 3 nm and 7 nm at 80 K. Numerical calculations within the envelope function framework have been performed to analyze the origin of the optical anisotropic forbidden transition. It is found that the optical anisotropy of this transition can be mainly attributed to indium segregation effect. The effect of uniaxial strain on in-plane optical anisotropy (IPOA) is also investigated. The IPOA of the forbidden transition changes little with strain, while that of the allowed transition shows a linear dependence on strain.PACS 78.66.Fd, 78.20.Bh, 78.20.Fm.

Anomalous piezoresistance effect in ultrastrained silicon nanowires

In this paper we demonstrate that under ultrahigh strain conditions p-type single crystal silicon nanowires possess an anomalous piezoresistance effect. The measurements were performed on vapor-liquid-solid (VLS) grown Si nanowires, monolithically integrated in a microelectro-mechanical loading module. The special setup enables the application of pure uniaxial tensile strain along the <111> growth direction of individual, 100 nm thick Si nanowires while simultaneously measuring the resistance of the nanowires. For low strain levels (nanowire elongation less than 0.8%), our measurements revealed the expected positive piezoresistance effect, whereas for ultrahigh strain levels a transition to anomalous negative piezoresistance was observed. For the maximum tensile strain of 3.5%, the resistance of the Si nanowires decreased by a factor of 10. Even at these high strain amplitudes, no fatigue failures are observed for several hundred loading cycles. The ability to fabricate single-crystal nanowires that are widely free of structural defects will it make possible to apply high strain without fracturing to other materials as well, therefore in any application where crystallinity and strain are important, the idea of making nanowires should be of a high value.

Enhanced quantum confinement due to nonuniform composition in alloy quantum dots

Strain and nanoscale variations in composition can significantly alter the electronic and optical properties of self-assembled alloy quantum systems. Using a combination of finite element and first-principles methods, we have developed an efficient and accurate technique to study the influence of strain and composition on the quantum confinement behavior in alloy quantum dots. Interestingly, we find that a nonuniform distribution of alloy components can lead to an enhanced confinement potential that allows a large quantum dot to behave electronically in a manner similar to a much smaller dot. The approach presented here provides a general means to quantitatively predict the influence of strain and composition variations on the performance characteristics of various small-scale alloy systems.

Electronic structure analysis for group III acceptors in Ge under stress considering screening effect and central-cell correction

We study theoretically the electronic structures of various group III acceptors in Ge under [001] stress, based on the effective-mass theory with a semi-empirical impurity potential which considers the q-dependent screening and the central-cell correction. An assignment is made for inter-level transition lines which were previously ignored or incorrectly assigned. In addition, our calculation can resolve crowding levels of final states of transition lines which have not been resolved by experimental techniques. The stress effect on the electronic structure can be understood by connecting with the composition of the states. Our results show that the binding energies decrease rapidly with the stress in the low-stress region, and for even-parity states they exhibit remarkable asymmetry in the stress dependence due to the large difference between the heavy-hole and the light-hole compositions. The acceptor states asymptotically approach a pure heavy-hole or light-hole state under high stress. In the limiting case of high stress, extra degeneracy appears. The central-cell correction may cause a significant chemical shift for even-parity states of nonisocoric acceptors. We also complete the assignment of the four line components into which the B line splits under stress. The newly assigned stress-dependent transition energies show excellent agreement with the experimental data for low stress. A justification is made for the applicability of our calculation scheme to the case of high stress.

Orientation Effects in Ballistic High-Strained P-type Si Nanowire FETs

In order to design and optimize high-sensitivity silicon nanowire-field-effect transistor (SiNW FET) pressure sensors, this paper investigates the effects of channel orientations and the uniaxial stress on the ballistic hole transport properties of a strongly quantized SiNW FET placed near the high stress regions of the pressure sensors. A discrete stress-dependent six-band k.p method is used for subband structure calculation, coupled to a two-dimensional Poisson solver for electrostatics. A semi-classical ballistic FET model is then used to evaluate the ballistic current-voltage characteristics of SiNW FETs with and without strain. Our results presented here indicate that [110] is the optimum orientation for the p-type SiNW FETs and sensors. For the ultra-scaled 2.2 nm square SiNW, due to the limit of strong quantum confinement, the effect of the uniaxial stress on the magnitude of ballistic drive current is too small to be considered, except for the [100] orientation. However, for larger 5 nm square SiNW transistors with various transport orientations, the uniaxial tensile stress obviously alters the ballistic performance, while the uniaxial compressive stress slightly changes the ballistic hole current. Furthermore, the competition of injection velocity and carrier density related to the effective hole masses is found to play a critical role in determining the performance of the nanotransistors.

Effects of lateral asymmetry on electronic structure of strained semiconductor nanorings in a magnetic field

The influence of lateral asymmetry on the electronic structure and optical transitions in elliptical strained InAs nanorings is analyzed in the presence of a perpendicular magnetic field. Two-dimensional rings are assumed to have elliptical inner and outer boundaries oriented in mutually orthogonal directions. The influence of the eccentricity of the ring on the energy levels is analyzed. For large eccentricity of the ring, we do not find any Aharonov-Bohm effect, in contrast to circular rings. Rather, the single-particle states of the electrons and the holes are localized as in two laterally coupled quantum dots formed in the lobes of the nanoring. Our work indicates that the control of shape is important for the existence of the Aharonov-Bohm effect in semiconductor nanorings.