Semiconductor Heterostructures Research Articles

Ternary nanocomposites of CdS/WO3/g-C3N4 for hydrogen production.

The sustainable rise in global warming and the consumption of fossil fuels considerably contribute to energy and environmental issues. To address these issues, semiconductor heterostructures can be used to generate clean energy sources as alternative energy sources and to reduce environmental impacts. Herein, we report the synthesis of a ternary semiconductor of the CdS/WO3/g-C3N4 (i.e. C-CNW) nanostructured composite for hydrogen production and dye degradation under visible-light irradiation. The structural properties of the prepared materials were studied using a series of investigational analyses. The 3C-CNW nanostructure photocatalyst exhibited faster malachite green (MG) dye photodegradation within 105 min and the highest hydrogen production rate is 868.23 μmol g-1 h-1 under visible light illumination. Moreover, the photocatalytic hydrogen production of the 3C-CNW nanostructure photocatalyst with various scavengers was analyzed. Its higher photocatalytic activity is ascribed to the Z-scheme mechanism, which induces rapid diffusion of photoinduced charges within the ternary photocatalyst with its optical bandgap. This proposed strategy is useful to improve photocatalysts that play a role in mitigating energy and environmental issues.

Leveraging machine learning to harness non-parabolic effects in semiconductor heterostructures

The engineering of optoelectronic devices is closely related to the dependency of quantum wells properties with structural parameters. Obtaining their properties relies on solving Schrödinger’s equation employing several degrees of approximation as the influence of band non-parabolicity effects. In general, the more sophisticated the approximations the longer the development cycle. To efficiently analyze the response of structures equipped with quantum wells, we propose using an artificial neural network trained with the results from the solution of Schrödinger’s equation. We observe an impressive agreement between the predicted quantum well’s eigenenergies and effective masses, in comparison to the results obtained from the iterative solution of the Schrödinger equation, for an extensive range of structures. The time spent to retrieve the properties of the structures using the artificial neural network was a fraction of the time spent with the solution of Schrödinger’s equation, enabling the neural network to be considered an effective tool in the development and improvement optoelectronic devices.

The effects of the electric-magnetic barrier on the spin transmission in the multibarrier semiconductor heterostructures

In the present paper, we study spin transmission in the multibarrier semiconductor heterostructures based on single particle effective mass approximation. These structures are double-barrier and triple-barrier semiconductor hetero-structures that a metallic ferromagnetic is deposited on them. Using Airy function and magnetic barriers approximated by delta function, we calculate transmission coefficient of tunneling electrons and spin polarization. Our results have shown that the parameters as the height and width of the electrical potential barrier, wave vector parallel to the barrier, applied bias voltage and magnetic field are effective parameters in determination of the transmission coefficient.

Electronic properties of graphene/CdX (X=S, Se, and Te) semiconductor heterostructure and a proposal of all-optical injection and detection of electron spins in graphene

Graphene was predicted to have a long spin relaxation time in the range of microseconds to milliseconds due to its weak spin-orbit coupling and hyperfine interaction. However, very short spin relaxation times were measured experimentally on the order of nanoseconds. Magnetic proximity effect from spin valves were considered strongly to influence the electronic and spin properties in graphene. Here, a graphene/nonmagnetic semiconductor CdX heterostructure is proposed to eliminate the magnetic effect and to fulfill optical injection of spin polarization. Based on the first-principles calculation within the density functional theory, we perform a systematic study on the structural, electronic properties and band structures of the heterostructures, and find that graphene on CdX (X = S, Se and Te) slabs preserves its linear Dirac band structure within the bandgap of CdX, and a built-in electric field occurs near the interface. The built-in electric field drives spin polarized electrons into graphene preferably. An optical detection scheme of spin relaxation time of graphene is proposed based on Faraday and Hanle effects. • A graphene/nonmagnetic semiconductor heterostructure constructed to eliminate strong magnetic effects on spin relaxation time of electrons in graphene. • First-principles calculations reveal Graphene on CdX (X = S, Se and Te) slabs preserves its linear Dirac band structure which locates within the bandgap of CdX. • A built-in electric field exists within interface, which favor spin polarized electrons transported into graphene. • A new scheme of all-optical injection and detection of electron spin is proposed to measure true long spin relaxation time of graphene.

XXVII Международный симпозиум Нанофизика и наноэлектроника", Нижний Новгород, 13-16 марта 2023 г. Формирование заряженных вакансий в анионной подрешетке AlAs

The vacancy formation dynamics in doped semicon- ductor heterostructures with quantum dots (QDs) formed in the AlAs anionic sublattice has been studied. A theoretical model that describes the effect of doping on the vacancy generation dynamics is constructed. It is shown that the generation of positively charged arsenic vacancies is more probable than the generation of neutral ones at high hole concentrations. On the other hand, at high electron concentrations, the formation of neutral arsenic vacancies is more efficient than that positively charged ones. It has been experimentally revealed that the vacancy-stimulated high- temperature diffusion of antimony is enhanced (suppressed) in p-(n-)-doped heterostructures with Al(Sb,As)/AlAs QDs.

Low temperature deposition of vanadium dioxide on III–V semiconductors and integration on mid-infrared quantum cascade lasers

We demonstrate low temperature deposition conditions for vanadium dioxide (VO2) phase change material by pulsed laser deposition, which are compatible with III–V semiconductors heterostructures typically used in optoelectronic applications. The characterizations of the VO2 coated thin films grown on GaAs show a 50% change in optical reflectivity in the mid-infrared range and a variation of electric conductivity of two orders of magnitude between the insulating (low temperature) and the metallic (high temperature) states. The transition temperature is estimated around 68 °C (341 K). We also study the functionalization of mid-infrared quantum cascade lasers (QCLs) (operating at wavelengths λ ∼ 7–8 μm) with VO2 layers, in view of engineering the laser emission properties with an integrated VO2 layer. We demonstrate QCLs that integrate a VO2 layer on the surface that interacts with the guided laser mode. A maximum operating temperature of 61 °C (334 K) has been measured.

Magnetic response of a two-dimensional viscous electron fluid

It has been established that the Coulomb interactions can transform the electron gas into a viscous fluid. This fluid is realized in a number of platforms, including graphene and two-dimensional semiconductor heterostructures. The defining characteristic of the electron fluid is the formation of layers of charge carriers that are in local thermodynamic equilibrium, as in classical fluids. In the presence of nonuniformities, whirlpools and nontrivial flow profiles are formed, which have been directly imaged in recent experiments. In this paper, we theoretically study the response of the electron fluid to localized magnetic fields. We find that the electric current is suppressed by viscous vortices in regions where magnetic field is sharply varying, causing strong transport signatures. Experimentally, our considerations are relevant since local magnetic fields can be applied to the system through implanting adatoms or embedding micromagnets in the top-gate. Our theory is essential for the characterization and future applications of electron fluids in hydrodynamic spin transport.

Lifetime of spin-orbit induced spin textures in a semiconductor heterostructure probed by quantum corrections to conductivity

The persistent spin helix (PSH) is the stable spin state protected by SU(2) spin-rotation symmetry. Long-lived spin textures, referred to as helical and homogeneous spin modes, emerge as a result of this symmetry. These textures are potential candidates for the development of quantum and topological phenomena as well as information carriers in semiconductors. To this end, revealing the lifetime of all spin modes is of great importance. We experimentally reveal the lifetime of both helical and homogeneous spin modes in the vicinity of the PSH state by fully electrical means through quantum corrections to the conductivity. In a (001)-grown GaAs/AlGaAs two-dimensional electron gas, we measure the weak antilocalization in the condition where Rashba and Dresselhaus spin-orbit (SO) interactions coexist. According to the latest theory on magnetoconductance [Kammermeier et al., Phys. Rev. B 104, 235430 (2021)], the Cooperon triplet mode in the quantum corrections can be decoupled into helical and homogeneous spin modes in the vicinity of the PSH state, which allows each mode lifetime to be determined from the quantum interference effect. By using a real-space simulation in tandem with the experiment, we were able to simultaneously evaluate the relaxation rates of the two spin modes. Our results show that the ratio of Rashba and Dresselhaus SO coefficients is modulated by the top gate and that this quadratically changes the relaxation rates of the helical and homogeneous spin modes, which is consistent with theoretical predictions. These findings pave the way for exploring electron spin textures in various bandgap materials from semiconductors to metals.

Hole spin manipulation in inhomogeneous and nonseparable electric fields

The usual models for electrical spin manipulation in semiconductor quantum dots assume that the confinement potential is separable in the three spatial dimensions and that the AC drive field is homogeneous. However, the electric field induced by the gates in quantum dot devices is not fully separable and displays significant inhomogeneities. Here, we address the electrical manipulation of hole spins in semiconductor heterostructures subject to inhomogeneous vertical electric fields and/or in-plane AC electric fields. We consider Ge quantum dots electrically confined in a Ge/GeSi quantum well as an illustration. We show that the lack of separability between the vertical and in-plane motions gives rise to an additional spin-orbit coupling mechanism (beyond the usual linear and cubic in momentum Rashba terms) that modulates the principal axes of the hole gyromagnetic g-matrix. This non-separability mechanism can be of the same order of magnitude as Rashba-type interactions, and enables spin manipulation when the magnetic field is applied in the plane of the heterostructure even if the dot is symmetric (disk-shaped). More generally, we show that Rabi oscillations in strongly patterned electric fields harness a variety of g-factor modulations. We discuss the implications for the design, modeling and understanding of hole spin qubit devices.

Large ferro–pyro–phototronic effect in 0.5Ba(Zr0.2Ti0.8)O3–0.5(Ba0.7Ca0.3)TiO3 thin films integrated on silicon for photodetection

AbstractCoupling together the ferroelectric, pyroelectric, and photovoltaic characteristics within a single material is a novel way to improve the performance of photodetectors. In this work, we take advantage of the triple multifunctionality shown by 0.5Ba(Zr0.2Ti0.8)O3–0.5(Ba0.7Ca0.3)TiO3 (BCZT), as demonstrated in an Al/Si/SiOx/BCZT/ITO thin‐film device. The Si/SiOx acts as an n‐type layer to form a metal–ferroelectric–insulator–semiconductor heterostructure with the BCZT, and with Al and ITO as electrodes. The photo‐response of the device, with excitation from a violet laser (405 nm wavelength), is carefully investigated, and it is shown that the photodetector performance is invariant with the chopper frequency owing to the pyro‐phototronic effect, which corresponds to the coupling together of the pyroelectric and photovoltaic responses. However, the photodetector performance was significantly better than that of the devices operating based only on the pyro‐phototronic effect by a factor of 4, due to the presence of ferroelectricity in the system. Thus, after a poling voltage of −15 V, for a laser power density of 230 mW/cm2 and at a chopper frequency of 400 Hz, optimized responsivity, detectivity, and sensitivity values of 13.1 mA/W, 1.7 × 1010 Jones, and 26.9, respectively, are achieved. Furthermore, ultrafast rise and fall times of 2.4 and 1.5 µs, respectively, are obtained, which are 35,000 and 36,000 times faster rise and fall responses, respectively, than previous reports of devices with the ferro–pyro–phototronic effect. This is understood based on the much faster ferroelectric switching in ferroelectric thin films owing to the predominant 180° domains in a single direction out of plane.

Strain Relaxation of Si/SiGe Heterostructures by a Geometric Monte Carlo Approach

Solid‐state‐based quantum technologies, such as electronic spin‐qubits, constitute a leading approach to the realization of quantum computation. Electronic spin‐qubits hosted in semiconductor heterostructures demand the highest crystalline quality, specifically with respect to the structure and formation of in‐plane misfit dislocations (MDs). Here, the formation mechanisms of MD networks in such Si/SiGe heterostructures are investigated. For this purpose, strained Si layers on relaxed Si0.7Ge0.3 with thicknesses above the critical thickness are grown by molecular beam epitaxy to allow for the formation of MDs. MDs are mapped out by electron channeling contrast imaging and the experimental results are compared to MD networks calculated by Monte Carlo simulations. The simulations show that the ratio between the threading dislocation density (TDD) and the average length of MDs is a constant for a given set of parameters, here 2.9 for sufficiently large dislocation statistics and simultaneous extension of MDs. Further, the fractal dimension of the MD network determined by the box‐counting method as an alternative quantity to characterize MD networks is introduced. This allows to infer the formation mechanism of MDs in real devices and hence the quantification of resulting average relaxation within the Si‐quantum well.

Micromagnets dramatically enhance effects of viscous hydrodynamic flow in a two-dimensional electron fluid

The hydrodynamic behavior of electron fluids in a certain range of temperatures and densities is well established in graphene and in two-dimensional semiconductor heterostructures. The hydrodynamic regime is intrinsically based on electron-electron interactions, and therefore it provides a unique opportunity to study electron correlations. Unfortunately, in simple longitudinal resistance measurements, the relative contribution of hydrodynamic effects to transport is rather small, especially at higher temperatures. Viscous hydrodynamic effects are masked by impurities, interaction with phonons, uncontrolled boundaries, and ballistic effects. This essentially limits the accuracy of measurements of electron viscosity. Fundamentally, what causes viscous friction in the electron fluid is the property of the flow called vorticity. In this paper, we propose to use micromagnets to increase the vorticity by orders of magnitude. Experimental realization of this proposal will bring electron hydrodynamics to a qualitatively new precision level, as well as opening a new way to characterize and externally control the electron fluid.

Band parameters of group III–V semiconductors in wurtzite structure

The properties of most III–V semiconductor materials in the wurtzite structure are not known because of their metastable character. However, recent advances in the growth of III–V wurtzite nanorods open new perspectives for applications. In this work, we present a systematic computational study of bulk wurtzite III–V semiconductors, using predictive ab initio methods, to provide a necessary base knowledge for studying the nanostructures. The most important physical properties of bulk systems, i.e., lattice constants, elasticity, spontaneous polarization, piezoelectricity, band structures, deformation potentials, and band offsets, have been studied. Comparison with the available experimental and theoretical data shows the high credibility of our results. Moreover, we provide a complete set of parameters for a six-band k⋅p model, which is widely used for simulating devices based on semiconductor heterostructures.

Three‐Dimensional Atomic‐Scale Tomography of Buried Semiconductor Heterointerfaces

AbstractAtom probes generate three‐dimensional atomic‐scale tomographies of material volumes corresponding to the size of modern‐day solid‐state devices. Here, the capabilities of atom probe tomography are evaluated to analyze buried interfaces in semiconductor heterostructures relevant for electronic and quantum devices. Employing brute‐force search, the current dominant reconstruction protocol to generate tomographic three‐dimensional images from Atom Probe data is advanced to its limits. Using Si/SiGe heterostructure for qubits as a model system, the authors show that it is possible to extract interface properties like roughness and width that agree with transmission electron microscopy observations on the sub‐nanometer scale in an automated and highly reproducible manner. The demonstrated approach is a versatile method for atomic‐scale characterization of buried interfaces in semiconductor heterostructures.

Demonstration and operation of quantum harmonic oscillators in an AlGaAs-GaAs heterostructure

The quantum harmonic oscillator (QHO), one of the most important and ubiquitous model systems in quantum mechanics, features equally spaced energy levels or eigenstates. Here we present a new class of nearly ideal QHOs formed by hydrogenic substitutional dopants in an AlGaAs/GaAs heterostructure. On the basis of model calculations, we demonstrate that, when a δ-doping Si donor substitutes the Ga/Al lattice site close to AlGaAs/GaAs heterointerface, a hydrogenic Si QHO, characterized by a restoring Coulomb force producing square law harmonic potential, is formed. This gives rise to QHO states with energy spacing of ∼8–9 meV. We experimentally confirm this proposal by utilizing gate tuning and measuring QHO states using an aluminum single-electron transistor (SET). A sharp and fast oscillation with period of ∼7–8 mV appears in addition to the regular Coulomb blockade (CB) oscillation with much larger period, for positive gate biases above 0.5 V. The observation of fast oscillation and its behavior is quantitatively consistent with our theoretical result, manifesting the harmonic motion of electrons from the QHO. Our results might establish a general principle to design, construct and manipulate QHOs in semiconductor heterostructures, opening future possibilities for their quantum applications.

Study of the Interface and Radial Dopant Position in Semiconductor Heterostructures Using X-ray Absorption Spectroscopy.

Two questions that remain a challenge in the field of colloidal doped core/shell nanomaterials of different morphologies are the nature of the interface and the radial location of the dopant ion due to the diffusion within the lattice. Using a model system of Cu-doped CdSe/CdS quantum dots, we develop an in-depth understanding of the extended X-ray absorption fine structure (EXAFS) spectra of the dopant and host atoms to address both issues. Our findings suggest that the interface is not sharp, in agreement with the nonstructural studies in the literature. Local structure analysis around the Cu dopant ion confirms that Cu drifts out from the core toward the outer region in the absence of the shell but stays mostly in the core after the formation of a sufficiently thick interfacial barrier (∼2 monolayers). This study highlights the significance of EXAFS spectroscopy in understanding the nature of the interface in nanomaterials.

Patterning alternate TiO2 and Cu2O strips on a conductive substrate as film photocatalyst for Z-scheme photocatalytic water splitting

Semiconductor heterostructures mediated by electrical conductors are very promising for Z-scheme photocatalytic water splitting. In contrast to conventional particulate heterostructures, alternate TiO2 and Cu2O film stripes patterned parallel on a fluorine-doped tin oxide (FTO) conductive substrate was fabricated as a model film photocatalyst to study the characteristics of the photogenerated charge transfer process. The Z-scheme transfer process with an effective transport distance of up to 5 μm occurs only in regions distant from the TiO2/Cu2O strip edges through the FTO substrate from the bottom. In contrast, the transfer of charge around their contact regions follows the conventional transfer process through the TiO2/Cu2O strip interface. These results indicate that the Z-scheme transfer process occurring in such a large region dominates the charge transfer processes in the TiO2/FTO/Cu2O pattern film heterostructure. Importantly, unlike the single component film, which is inactive for photocatalytic overall water splitting, the modified TiO2/Cu2O pattern film can induce photocatalytic overall water splitting at a stoichiometric H2/O2 ratio close to 2:1. These findings have significant implications in designing efficient heterostructures by employing a Z-scheme charge transfer process.

Direct measurement of band offsets on selective area grown In0.53Ga0.47As/InP heterojunction with multiple probe scanning tunneling microscopy

The knowledge of the band alignment in semiconductor heterostructures is crucial, as it governs carrier confinement with many impacts on the performances of devices. By controlling the direction of the current flow in in-plane In0.53Ga0.47As/InP heterostructure nanowires, either horizontally along the nanowires or vertically into the InP substrate with low temperature multiple-probe tunneling spectroscopy, a direct measurement of the band offsets at the buried In0.53Ga0.47As/InP heterointerface is performed. Despite the unavoidable processing steps involved in selective area epitaxy, conduction and valence band offsets of 0.21 ± 0.01 and 0.40 ± 0.01 eV are, respectively, found, indicating the formation of an interface with a quality comparable to two-dimensional In0.53Ga0.47As/InP heterostructures.

Reconfiguring band-edge states and charge distribution of organic semiconductor-incorporated 2D perovskites via pressure gating.

Two-dimensional (2D) semiconductor heterostructures are key building blocks for many electronic and optoelectronic devices. Reconfiguring the band-edge states and modulating their interplay with charge carriers at the interface in a continuous manner have long been sought yet are challenging. Here, using organic semiconductor-incorporated 2D halide perovskites as the model system, we realize the manipulation of band-edge states and charge distribution via mechanical-rather than chemical or thermal-regulation. Compression induces band-alignment switching and charge redistribution due to the different pressure responses of organic and inorganic building blocks, giving controllable emission properties of 2D perovskites. We propose and demonstrate a "pressure gating" strategy that enables the control of multiple emission states within a single material. We also reveal that band-alignment transition at the organic-inorganic interface is intrinsically not well resolved at room temperature owing to the thermally activated transfer and shuffling of band-edge carriers. This work provides important fundamental insights into the energetics and carrier dynamics of hybrid semiconductor heterostructures.

Analysis of Carrier Transport in Quantum Dot/Metal‐Oxide Phototransistors via Light‐Mediated Interfacial Modeling

AbstractQuantum dot (QD) hybrid phototransistors based on high‐mobility channel semiconductors have attracted great interest due to their outstanding photonic characteristics with diverse and broadband optoelectronic functionalities. However, difficulties for precise control of electronic coupling between QD particles and lack of QD/semiconductor interface analysis have limited the use of these platforms into practical optoelectronic applications. Here, the authors report a new strategy to high‐performance phototransistors based on CdSe QD/amorphous oxide semiconductor heterostructures using trap‐reduced chelating chalcometallate surface ligands by optimizing the QD layer thickness and interface engineering to enhance photogenerated charge transfer properties. Furthermore, based on performance parameters extracted from experimental structure and electrical characteristics, Technology Computer‐Aided Design (TCAD) modeling reveals carrier flow between the QDs in the semiconductor layer in agreement with various spectroscopic data. These modeling results indicate that interface engineering of QD with chalcometallate ligand results in very efficient photo‐induced charge transfer characteristics. Thus, high‐performance chalcometallate ligand QD‐based phototransistors are successfully realized with enhanced responsivity (2.12 × 104 A W−1) and photodetectivity (2.74 × 1016 Jones) compared with conventional monodentate ligand functionalized (SCN−) QD‐based devices (8.79 × 103 A W−1 and 8.8 × 1010 Jones, respectively). This is the demonstration of systematic analysis of QD‐based hetero‐structured phototransistors with light‐mediated interfacial physical modeling and electrochemical measurement.