Lower bound on the spread of valley splitting in Si/SiGe quantum wells induced by atomic rearrangement at the interface

The achievement of universal quantum computing critically relies on scalability.However,ensuring the necessary uniformity for scalable silicon electron spin qubits poses a significant challenge due to the considerable fluctuations in valley splitting energy(E_(VS))across quantum dot arrays,which impede the initialization of qubit systems comprising multiple spins and give rise to spin–valley entanglement resulting in the loss of spin information.These E_(VS)fluctuations have been attributed to variations in the in-plane averaged alloy concentration along the confinement direction of Si/SiGe quantum wells.In this study,employing atomistic pseudopotential calculations,we unveil a significant spectrum of E_(VS)even in the absence of such concentration fluctuations.This spectrum represents the lower limit of the wide range of E_(VS)observed in numerous Si/SiGe quantum devices.By constructing simplified interface atomic step models,we analytically demonstrate that the lower bound of the E_(VS)spread originates from the in-plane random distribution of Si and Ge atoms within SiGe barriers——an inherent characteristic that has been previously overlooked.Additionally,we propose an interface engineering approach to mitigate the in-plane randomness-induced fluctuations in E_(VS)by inserting a few monolayers of pure Ge barrier at the Si/SiGe interface.Our findings provide valuable insights into the critical role of in-plane randomness in determining E_(VS)in Si/SiGe quantum devices and offer reliable methods to enhance the feasibility of scalable Si-based spin qubits.

Direct observation of the carrier transport process in InGaN quantum wells with a pn-junction

A new mechanism of light-to-electricity conversion that uses InGaN/GaN QWs with a p-n junction is reported.According to the well established light-to-electricity conversion theory,quantum wells(QWs) cannot be used in solar cells and photodetectors because the photogenerated carriers in QWs usually relax to ground energy levels,owing to quantum confinement,and cannot form a photocurrent.We observe directly that more than 95% of the photoexcited carriers escape from InGaN/GaN QWs to generate a photocurrent,indicating that the thermionic emission and tunneling processes proposed previously cannot explain carriers escaping from QWs.We show that photoexcited carriers can escape directly from the QWs when the device is under working conditions.Our finding challenges the current theory and demonstrates a new prospect for developing highly efficient solar cells and photodetectors.

Dispersion of exciton-polariton based on ZnO/MgZnO quantum wells at room temperature

We report observation of dispersion for coupled exciton-polariton in a plate microcavity combining with ZnO/MgZnO multi-quantum well (QW) at room temperature. Benefited from the large exciton binding energy and giant oscillator strength, the room-temperature Rabi splitting energy can be enhanced to be as large as 60 meV. The results of excitonic polariton dispersion can be well described using the coupling wave model. It is demonstrated that mode modification between polariton branches allowing, just by controlling the pumping location, to tune the photonic fraction in the different detuning can be investigated comprehensively. Our results present a direct observation of the exciton-polariton dispersions based on two-dimensional oxide semiconductor quantum wells, thus provide a feasible road for coupling of exciton with photon and pave the way for realizing novel polariton-type optoelectronic devices.

Raman spectrum study of δ-doped GaAs/AlAs multiple-quantum wells

Three samples of GaAs/AlAs multiple-quantum wells with different quantum well widths and δ-doped with Be acceptors at the well center were grown on(100) Ga As substrates by molecular beam epitaxy. Polarized Raman spectra were recorded on the three samples at temperatures in a range of 4-50 K in a backscattering configuration. The two branches of coupled modes due to the interaction of the hole intersubband transitions and the quantum-well longitudinal optical(LO) phonon were observed clearly. The evaluation formalism of the Green function was employed and each lineshape of the Raman spectrum of the coupled modes was simulated. The dependence of the peak position of Raman shifts of the two coupled modes as well as the quantum-well LO phonon on the quantum-well size and measured temperature were given, and the coupling interaction mechanism between the hole subband transitions and the quantum-well LO phonon was researched.

Improved thermal property of strained InGaAlAs/AlGaAs quantum wells for 808-nm vertical cavity surface emitting lasers

The 808-nm vertical cavity surface emitting laser(VCSEL) with strained In_(0.13)Ga_(0.75)Al_(0.12)As/Al_(0.3)Ga_(0.7)As quantum wells is designed and fabricated. Compared with the VCSELs with Al_(0.05)Ga_(0.95)As/Al_(0.3)Ga_(0.7)As quantum wells, the VCSEL with strained In_(0.13)Ga_(0.75)Al_(0.12)As/Al_(0.3)Ga_(0.7)As quantum wells is demonstrated to possess higher power conversion efficiency(PCE) and better temperature stability. The maximum PCE of 43.8% for 10-μm VCSEL is achieved at an ambient temperature of 30°C. The size-dependent thermal characteristics are also analyzed by characterizing the spectral power and output power. It demonstrates that small oxide-aperture VCSELs are advantageous for temperature-stable performance.

Flexible,stretchable,and transparent InGaN/GaN multiple quantum wells/polyacrylamide hydrogel-based light emitting diodes

Visualization is a direct,efficient,and simple interface method to realize the interaction between human and machine,whereas the flexible display unit,as the major bottleneck,still deeply hinders the advances of wearable and virtual reality devices.To obtain flexible optoelectronic devices,one of the effective methods is to transfer a high-efficient and long-lifetime inorganic optoelectronic film from its rigid epitaxial substrate to a foreign flexible/soft substrate.Additionally,piezo-phototronic effect is a fundamental theory for guiding the design of flexible optoelectronic devices.Herein,we demonstrate a flexible,stretchable,and transparent InGaN/GaN multiple quantum wells(MQWs)/polyacrylamide(PAAM)hydrogel-based light emitting diode coupling with the piezo-phototronic effect.The quantum well energy band and integrated luminous intensity(increased by more than 31.3%)are significantly modulated by external mechanical stimuli in the device.Benefiting from the small Young's modulus of hydrogel and weak Van der Waals force,the composite film can endure an extreme tensile condition of about 21.1%stretching with negligible tensile strains transmitted to the InGaN/GaN MQWs.And the stable photoluminescence characteristics can be observed.Moreover,the hydrogen-bond adsorption and excellent transparency of the hydrogel substrate greatly facilitate the packaging and luminescence of the optoelectronic device.And thus,such a novel integration scheme of inorganic semiconductor materials and organic hydrogel materials would help to guide the robust stretchable optoelectronic devices,and show great potential in emerging wearable devices and virtual reality applications.

High-Performance Optical Modulators Based on Potential- Tailored Quantum Wells

The five -layer asymmetric coupled quantum well ( FACQW) , which is one of the potential -tailored quantum wells, is expected to show very large electrorefrac live index cha nge in a wideband transpar ency region far from the absorption edge. Characteristics of the FACQW and its application to compact, ultrafast , low voltage optical modul ators and switches are dis cussed.

The chemistry and physics of organic-inorganic hybrid perovskite quantum wells

Organic-inorganic hybrid two dimensional(2D)lead halide perovskites(LHPs)are tunable quantum wells that exhibit a set of intriguing structural and physical properties including soft and dynamic lattices,organic-inorganic epitaxial heterointerfaces,quantum and dielectric confinements,strong light-matter interactions,and large spin-orbit coupling,which enable promising perspectives for optoelectronics,ferroelectrics,and spintronics.While the properties of 2D LHPs bear some resemblance of the3D LHPs,they are often drastically altered due to the reduced dimensionality and the complex interactions between organic and inorganic components.In this review,we discuss the influences of the reduced dimensionality and the organic-inorganic interplays on the structural stability and distortion of the inorganic lattices,inversion symmetry of the crystal structure,electronic band structures,excitonic physics,and carrier-phonon interactions in 2D LHPs.An emphasis is placed on the relationships between the crystal structures and photophysical properties.Future perspectives on the opportunities of hybrid quantum wells are provided.

Nonpolar Al_(x)Ga_(1−x)N/Al_(y)Ga_(1−y)N multiple quantum wells on GaN nanowire for UV emission

Nonpolar m-plane AlGaN offers the advantage of polarization-free multiple quantum wells(MQWs)for ultraviolet(UV)emission and can be achieved on the sidewalls of selective area grown GaN nanowires.We reveal that the growth of AlGaN on GaN nanowires by metal organic chemical vapor deposition(MOCVD)is driven by vapor-phase diffusion,and consequently puts a limit on the pitch of nanowire array due to shadowing effect.An insight into the difficulty of achieving metal-polar AlGaN nanowire by selective area growth(SAG)in MOCVD is also provided and can be attributed to the strong tendency to form pyramidal structure due to a very small growth rate of{1011}semipolar planes compared to(0001)c-plane.The nonpolar m-plane sidewalls of GaN nanowires obtained via SAG provides an excellent platform for growth of nonpolar AlGaN MQWs.UV emission from mplane Al_(x)Ga_(1−x)N/Al_(y)Ga_(1−y)N MQWs grown on sidewalls of dislocation-free GaN nanowire is demonstrated in the wavelength range of 318–343 nm.

Individually resolved luminescence from closely stacked GaN/AlN quantum wells

Investigating closely stacked GaN/AlN multiple quantum wells(MQWs)by means of cathodoluminescence spectroscopy directly performed in a scanning transmission electron microscope,we have reached an ultimate spatial resolution ofσCL=1.8 nm.The pseudomorphically grown MQWs with high interface quality emit in the deep ultraviolet spectral range.Demonstrating the capability of resolving the 10.8 nm separated,ultra-thin quantum wells,a cathodoluminescence profile was taken across individual ones.Applying a diffusion model of excitons generated by a Gaussian-broadened electron probe,the spatial resolution of cathodoluminescence down to the free exciton Bohr radius scale has been determined.

An Exactly Solvable Algebraic Model for Single Quantum Well Treatments

We propose an algebraic model, presenting individual contributions separately in the system of interest, for the exact solutions of one-dimensional Poisson-Schr?dinger equations used generally in semiconductor device simulations. The model presented here reveals an interesting relation between the corresponding Poisson and Schr?dinger equation for the physical structure considered, which leads to closed solutions without solving the required electrostatic equation.

Single-mode GaSb-based laterally coupled distributed-feedback laser for CO_(2)gas detection

We report on a GaSb-based laterally coupled distributed feedback(LC-DFB)laser with Cr gratings operating at 2004 nm for CO_(2)detection application.Butterfly packaged with single-mode fiber pigtailed,the laser diode operates in the continuous-wave mode in a temperature range from-10℃to 60℃,with a maximum output power of 2 mW and a maximum side-mode suppression ratio over 30 dB.Wavelength-modulated absorption spectroscopy of CO_(2)demonstrates the applicability of the LC-DFB laser to tunable diode laser absorption spectroscopy.Furthermore,the diode junction temperature,which is measured by using the wavelength shift method,exhibits a maximum value of 17℃in the single-mode operation range.

Enhancing performance of GaN-based LDs by using GaN/InGaN asymmetric lower waveguide layers

The effects of Ga N/In Ga N asymmetric lower waveguide(LWG)layers on photoelectrical properties of In Ga N multiple quantum well laser diodes(LDs)with an emission wavelength of around 416 nm are theoretically investigated by tuning the thickness and the indium content of In Ga N insertion layer(In Ga N-IL)between the Ga N lower waveguide layer and the quantum wells,which is achieved with the Crosslight Device Simulation Software(PIC3D,Crosslight Software Inc.).The optimal thickness and the indium content of the In Ga N-IL in lower waveguide layers are found to be 300 nm and 4%,respectively.The thickness of In Ga N-IL predominantly affects the output power and the optical field distribution in comparison with the indium content,and the highest output power is achieved to be 1.25 times that of the reference structure(symmetric Ga N waveguide),which is attributed to the reduced optical absorption loss as well as the concentrated optical field nearby quantum wells.Furthermore,when the thickness and indium content of In Ga N-IL both reach a higher level,the performance of asymmetric quantum wells LDs will be weakened rapidly due to the obvious decrease of optical confinement factor(OCF)related to the concentrated optical field in the lower waveguide.

A contrivance of 277 nm DUV LD with B0.313Ga0.687N/B0.40Ga0.60N QWs and AlxGa1–xN heterojunction grown on AlN substrate

In this paper,an ultraviolet C-band laser diode lasing at 277 nm composed of B0.313Ga0.687N/B0.40Ga0.60N QW/QB heterostructure on Mg and Si-doped AlxGa1-xN layers was designed,as well as a lowest reported substitutional accepter and donor concentration up to NA=5.0×10^17 cm^-3 and ND=9.0×10^16 cm^-3 for deep ultraviolet lasing was achieved.The structure was assumed to be grown over bulk AIN substrate and operate under a continuous wave at room temperature.Although there is an emphasizing of the suitability for using boron nitride wide band gap in the deep ultraviolet region,there is still a shortage of investigation about the ternary BGaN in aluminum-rich AIGaN alloys.Based on the simulation,an average local gain in quantum wells of 1946 cm^-1,the maximum emitted power of 2.4 W,the threshold current of 500 mA,a slope efficiency of 1.91 W/A as well as an average DC resistance for the V-I curve of(0.336Ω)had been observed.Along with an investigation regarding different EBL,designs were included with tapered and inverse tapered structure.Therefore,it had been found a good agreement with the published results for tapered EBL design,with an overweighting for a proposed inverse tapered EBL design.

A crossover from Efros–Shklovskii hopping to activated transport in a GaAs two-dimensional hole system at low temperatures

In this paper, we discuss low-temperature hopping-conductivity behavior in the insulating phase, in the absence of a magnetic field. We conduct a theoretical study of the crossover from hopping to activated transport in a GaAs two-dimensional hole system at low temperatures, finding that a crossover takes place from the Efros-Shklovskii variable-range hopping(VRH)regime to an activated regime in this system. This conductivity behavior in p-GaAs quantum wells is qualitatively consistent with the laws laid down in theories of localized electron interactions. Given sufficiently strong interactions, the holes in the localized states are able to hop collectively.

Novel Understanding of Electron States Architecture and Its Dimensionality in Semiconductors

Some important insights into the electron-states-architecture (ESA) and its dimensionality (from 3 to 0) in a semiconductor (or generally crystalline) material are obtained. The self-consistency of the set of density of states (DOS) expressions with different dimensionalities is remediated through the clarification and rearrangement of the wave-function boundary conditions for working out the eigenvalues in the wave vector space. The actually too roughly observed and theoretically unpredicted critical points for the dimensionality transitions referring to the integer ones are revealed upon an unusual assumption of the intrinsic energy-level dispersion (ELD). The ELD based quantitative physical model had been established on an immediate instinct at the very beginning and has been properly modified afterwards. The uncertainty regarding the relationship between the de Broglie wavelength of electrons and the dimensionality transitions, seeming somewhat mysterious before, is consequentially eliminated. The effect of the material dimensions on the ELD width is also predicted and has been included in the model. The continuous evolution of the ESA dimensionality is convincingly and comprehensively interpreted and thus the area of the fractional ESA dimensionalities is opened. Another new assumption of the spatial extension shrinkage (SES) closely related to the ELD has also been made and thus the understanding of the behavior of an electron or, in a general sense, a particle has become more comprehensive. This work would manifest itself a new basis for further development of nanoheterostructures (or low dimensional heterostructures including the quantum wells, quantum wires, quantum dots and especially the hetero-dimensional structures). Expected should also be the possible inventions of some novel electronic and optoelectronic devices. More basically, it leads to a new quantum mechanical picture, the essential modifications of Schrödinger equation and Newtonian equation that give rise to a full cosmic-scope picture, and a super-low-speed relativity assumption.

Efficiency droop in InGaN/GaN-based LEDs with a gradually varying In composition in each InGaN well layer

Temperature-dependent and driving current-dependent electroluminescence spectra of two different InGaN/GaN multiple quantum well structures SA and SB are investigated,with the In composition in each well layer(WL)along the growth direction progressively increasing for SA and progressively decreasing for SB.The results show that SB exhibits an improved efficiency droop compared with SA.This phenomenon can be explained as follows:owing to the difference in growth pattern of the WL between these two samples,the terminal region of the WL in SB contains fewer In atoms than in SA,and therefore the former undergoes less In volatilization than the latter during the waiting period required for warming-up due to the difference in the growth temperature between well and barrier layers.This results in SB having a deeper triangular-shaped potential well in its WL than SA,which strongly confines the carriers to the initial region of the WL to prevent them from leaking to the p-GaN side,thus improving the efficiency droop.Moreover,the improvement in the efficiency droop for SB is also partly attributed to its stronger Coulomb screening effect and carrier localization effect.

Piezoelectric constant temperature dependence in strained [111]-oriented zinc-blende MQW-SOAs

Here,we present a study of the effective piezoelectric constant(e_(14_(e)))temperature dependence in strained[111]-oriented zinc-blende quantum wells(QWs)embedded within a semiconductor optical amplifier(SOA).We determined e_(14_(e)) using a method that was insensitive to the segregation phenomenon and to the temperature dependence of the bandgap energy,which required neither fitting parameters nor temperature-dependent expressions for energy and out-of-plane effective masses of electrons and heavy holes.An e_(14_(e))=−0.0534±0.0040 C·m^(−2) at 23°C was obtained for an SOA with 1.2 nm[111]-oriented strained In0.687Ga0.313As/In0.807Ga0.193As0.304P0.696 QWs.Unlike previously published research,where e_(14_(e)) magnitude increased as temperature rised,we extracted an e_(14_(e)) magnitude that decreased as temperature increased.