Quantum-well States Research Articles

Observations of two-dimensional electron gases in AlGaN/GaN high-electron-mobility transistors using up-converted photoluminescence excitation.

Up-converted photoluminescence excitation (UPLE) spectra of AlGaN/GaN high-electron-mobility transistors (HEMTs) grown on Si substrates have been investigated. Based on the temperature dependence of UPLE, the 3.335-eV excitation peak is attributed to the two-dimensional electron gases (2DEGs) in the AlGaN/GaN heterostructure. A two-step two-photon absorption process through real intermediate quantum-well states is suggested to be responsible for the up-converted luminescence in the AlGaN/GaN HEMTs.

Substantial Energy Band Modulation of Semiconducting Hexagonal GaTe Quantum Wells by Layer Thickness and Mirror Twin Boundaries.

Exploring emerging two-dimensional (2D) van der Waals (vdW) semiconducting materials and precisely tuning their electronic properties at the atomic level have long been recognized as crucial issues for developing their high-end electronic and optoelectronic applications. As a III-VI semiconductor, ultrathin layered hexagonal GaTe (h-GaTe) remains unexplored in terms of its intrinsic electronic properties and band engineering strategies. Herein, we report the successful synthesis of ultrathin h-GaTe layers on a selected graphene/SiC(0001) substrate, via molecular beam epitaxy (MBE). The widely tunable quasiparticle band gaps (∼2.60-1.55 eV), as well as the vdW quantum well states (QWSs) that can be strictly counted by the layer numbers, are well characterized by onsite scanning tunneling microscopy/spectroscopy (STM/STS), and their origins are clearly addressed by density functional theory (DFT) calculations. More intriguingly, distinctive 8|8E and 4|4P (Ga) mirror twin boundaries (MTBs) are identified in the ultrathin h-GaTe flakes, which can induce decreased band gaps and prominent p-doping effects. This work should deepen our understanding on the electronic tunability of 2D III-VI semiconductors by thickness control and line defect engineering, which may hold promise for fabricating atomic-scale vertical and lateral homojunctions toward ultrascaled electronics and optoelectronics.

Nematic Metal in a Multivalley Electron Gas: Variational MonteCarlo Analysis and Application to AlAs.

The two-dimensional electron gas is of fundamental importance in quantum many-body physics. We study a minimal extension of this model with C_{4} (as opposed to full rotational) symmetry and an electronic dispersion with two valleys with anisotropic effective masses. Electrons in our model interact via Coulomb repulsion, screened by distant metallic gates. Using variational MonteCarlo simulations, we find a broad intermediate range of densities with a metallic valley-polarized, spin-unpolarized ground state. Our results are of direct relevance to the recently discovered "nematic" state in AlAs quantum wells. For the effective mass anisotropy relevant to this system, m_{x}/m_{y}≈5.2, we obtain a transition from an anisotropic metal to a valley-polarized metal at r_{s}≈12 (where r_{s} is the dimensionless Wigner-Seitz radius). At still lower densities, we find a (possibly metastable) valley and spin-polarized state with a reduced electronic anisotropy.

Polarized photoluminescence and g-factor in an inhomogeneous ensemble of quantum dots in magnetic fields

In this work, polarized photoluminescence caused by quasi-equilibrium spin orientation of excitons at Zeeman sublevels in an ensemble of quantum dots of different sizes is studied theoretically. The following unexpected results are found: (i) the splitting of photoluminescence bands in a magnetic field in an ensemble of quantum dots is much larger than the Zeeman splitting of exciton levels in a single dot, (ii) contrary to the Boltzmann distribution the low-energy luminescence band has a lower intensity than the high-energy band, (iii) the sign of the circular polarization of photoluminescence changes along the contour of the bands, (iv) a universal formula for the dependence of the exciton g-factor on the quantum dot size is obtained. All of the above features of the polarized luminescence spectra are explained by variations of the exciton g-factor when its quantization energy changes. The results obtained are applicable not only to ensembles of quantum dots, but also to any ensemble of localized states: excitons in quantum wells under conditions of well-width fluctuations, impurity states in quantum wells, etc.

Combined effects of temperature and confinement on the Shannon entropy of two-dimensional hydrogenic impurity states in the GaAs semiconductor quantum well

Understanding the quantum properties of two-dimensional (2-D) hydrogenic impurity states in the GaAs quantum well is essential for their applications in the semiconductor devices. Here, we report the combined effects of temperature and quantum confienment on the Shannon entropy of 2-D hydrogenic impurity states in the GaAs quantum well. First, by calculating the position and momentum space Shannon entropy (Sr and Sp), as well as the Shannon entropy sum (St), for the hydrogenic impurity states in the GaAs quantum well at room temperature, we show fascinating quantum confinement properties in the two conjugated spaces. Then, the effects of temperature on the Shannon entropy for different hydrogenic impurity states are examined and compared. Intriguingly, we discover that the Shannon entropy sum exhibits the property of translation invariance under the influence of temperature. Finally, with the radius of the quantum well fixed at a constant, we observe that the value of the Shannon entropy for a given quantum state is highly sensitive to the variations in temperature. While the size of circular quantum well is constrained by design parameters, such as the fabrication process and intended application, temperature adjustment emerges as a viable means to control the quantum properties of 2-D hydrogenic impurities in the quantum well. This research presents a pioneering method that employs temperature as a versatile tool for modulating the Shannon entropy of hydrogenic impurity states in the quantum well, and has potential applications in the field of semiconductor physics, material physics, chemical physics, etc.

Epitaxial growth of Si thin films with hexagonal close-packed structures on metal substrates.

We have studied the epitaxial growth of Si thin films on the Cd(0001) surface using low-temperature scanning tunneling microscopy. When deposited at low temperatures (100K), Si atoms form dendritic islands with triangular shapes, indicating the existence of anisotropic edge diffusion in the process of Si film growth. After annealing to elevated temperatures, the triangular dendritic Si islands become hexagonal compact islands. Moreover, the 2D Si islands located on two different substrate terraces exhibit different heights due to the influence of quantum-well states in Cd(0001) films. Based on high-resolution scanning tunneling microscopy images, it is observed that the first, second, and third Si layers show the pseudomorphic 1 × 1 structure. In particular, the first and second layer islands reveal the opposite triangles, indicating the hexagonal close-packed stacking of Si atoms. These results provide important information for the growth of pristine Si films on metal substrates and the understanding of Si-metal interaction.

Quantum Plasmonics in Sub-Atom-Thick Optical Slots.

We show using time-dependent density functional theory (TDDFT) that light can be confined into slot waveguide modes residing between individual atomic layers of coinage metals, such as gold. As the top atomic monolayer lifts a few Å off the underlying bulk Au (111), ab initio electronic structure calculations show that for gaps >1.5 Å, visible light squeezes inside the empty slot underneath, giving optical field distributions 2 Å thick, less than the atomic diameter. Paradoxically classical electromagnetic models are also able to reproduce the resulting dispersion for these subatomic slot modes, where light reaches in-plane wavevectors ∼2 nm-1 and slows to <10-2c. We explain the success of these classical dispersion models for gaps ≥1.5 Å due to a quantum-well state forming in the lifted monolayer in the vicinity of the Fermi level. This extreme trapping of light may explain transient "flare" emission from plasmonic cavities where Raman scattering of metal electrons is greatly enhanced when subatomic slot confinement occurs. Such atomic restructuring of Au under illumination is relevant to many fields, from photocatalysis and molecular electronics to plasmonics and quantum optics.

Probing conduction band offsets and confined states at GaAs/GaAsNBi heterointerfaces

We probe the conduction-band offsets (CBOs) and confined states at GaAs/GaAsNBi quantum wells (QWs). Using a combination of capacitance–voltage (C–V) measurements and self-consistent Schrödinger–Poisson simulations based on the effective mass approximation, we identify an N-fraction dependent increase in CBO, consistent with trends predicted by the band anti-crossing model. Using the computed confined electron states in conjunction with photoluminescence spectroscopy data, we show that N mainly influences the conduction band and confined electron states, with a relatively small effect on the valence band and confined hole states in the quaternary QWs. This work provides important insight toward tailoring CBO and confined electron energies, both needed for optimizing infrared optoelectronic devices.

Thermodynamic property of one-dimensional hydrogenic impurity in Nitride semiconductor quantum well

ABSTRACT The thermodynamic properties of one-dimensional (1-D) hydrogenic impurity in Nitride semiconductor quantum well have been investigated. Firstly, using a combination of the linear variational approach and the effective mass approximation, we obtain the eigen-energy of hydrogenic impurity states in AlN, GaN and InN quantum wells. Subsequently, we explore the quantum size effect on the thermodynamic properties of these low-dimensional semiconductor structures, revealing some novel and intriguing phenomena. It is found as the width of the quantum well is increased, the energy of hydrogenic impurity gets decreased, however, the thermodynamic parameters, such as the average energy, entropy and heat capacity of the hydrogenic impurity become increased. The physical origins of these peculiar phenomena are attributed to the competitive mechanism between the quantum confinement effect and Coulombic interaction between the electron and impurity. Finally, by conducting a comprehensive comparison of the thermodynamic parameters among GaN, InN, and AlN quantum wells, we propose an innovative strategy for modulating the thermodynamic properties of these low-dimensional semiconductor structures. This manipulation can be achieved not only through external factors such as temperature and quantum well size, but also by altering internal variables like the effective mass and relative dielectric constant of the semiconductor material. This study not only advances our understanding of the thermodynamic properties of hydrogenic impurities in semiconductor quantum wells, but also provides some theoretical guidance for practical applications in fields such as semiconductor physics, condensed matter physics, chemical physics, etc.

Resonant optical reflection from a GaN/(Al,Ga)N excitonic Bragg structure

We experimentally demonstrate the formation of a superradiant optical mode in the room-temperature reflection spectra from a resonant Bragg structure composed of 30 equidistant GaN quantum wells separated by (Al,Ga)N barriers. The mode arises when the condition of the Bragg diffraction is fulfilled at the wavelength corresponding to the energy of the quasi-two-dimensional excitons in the quantum wells. It manifests itself as a significant increase in the amplitude and a change in the shape of the resonant optical reflection due to the electromagnetic coupling of the excitons. By modeling of the optical spectra, we evaluate the radiative and non-radiative broadening parameters of the excitonic states in the GaN quantum wells, which appear to be 0.4 ± 0.02 and 40 ± 5 meV, correspondingly, for the resonant exciton energy of 3.605 eV. The resonant Bragg structure based on the periodic sequence of the GaN quantum wells demonstrates an efficient coupling of photons and excitons at room temperature, which makes it promising for device applications.

Analytical estimation of lifetime of quasi-bound states in iii-v semiconductors quantum well

The lifetime of electrons in the Quasi Bound States in a quantum well formed by III-V semiconductors has been calculated numerically by several researchers in the past. In this work, we obtain an analytical expression for the lifetime by finding the poles of the scattering matrix in small-width approximation. Airy functions which are solutions of the Schrödinger equation for triangular potentials are expanded asymptotically for large arguments and in power series for small arguments. A scattering problem for the triangular potential well is solved with the help of mixed boundary conditions to derive the expression for the decay width that further gives the net tunneling current from the quantum well. Heterostructures of III-V materials are modeled with triangular wells whose experimental measurements are in close approximation with the theoretical calculations presented in this paper. The analytically calculated decay width in this work is also compared with the reported values by different numerical methods and found in close agreement with them.

Probing semiconductor quantum well qubits and associated Shannon entropy using semi-relativistic quantum mechanics

Electron states in GaAs, GaN and AlN quantum wells are studied by solving a semi-relativistic wave equation within the effective mass approximation. The quantum states are in turn used to probe the properties of two-level qubits formed in the different quantum wells at various temperatures. Results indicated that the period of oscillation between the quantum states increases with increasing width of the quantum wells, with AlN having the longest period and shortest for GaAs. Transition rates were also studied, since their product with the period of oscillation yield important information concerning the feasibility of carrying out a quantum computation. This product is equivalent to the ratio of the period of oscillation between states to the lifetime of an electron in an excited state. From the results, GaAs quantum wells may be preferable as they have the lowest ratio compared with the other quantum wells of other materials. AlN has the highest ratio of the three semiconductors considered here. Shannon entropy in the different quantum wells was studied also. It was found that the entropy in GaAs quantum wells varies rapidly through the passage of time, while those of GaN and AlN vary relatively slowly.

M-plane GaN terahertz quantum cascade laser structure design and doping effect for resonant-phonon and phonon-scattering-injection schemes

Non-polar m-plane GaN terahertz quantum cascade laser (THz-QCL) structures have been studied. One is traditional three-well resonant-phonon (RP) design scheme. The other is two-well phonon scattering injection (PSI) design scheme. The peak gains of 41.8 and 44.2 cm−1 have been obtained at 8.2 and 7.7 THz respectively at 300 K according to the self-consistent non-equilibrium Green’s function calculation. Different from the usual GaAs two-well design, the upper and lower lasing levels are both ground states in the GaN quantum wells for the PSI scheme, mitigating the severe broadening effect for the excited states in GaN. To guide the fabrication of such devices, the doping effect on the peak gain has been analyzed. The two designs have demonstrated distinct doping density dependence and it is mainly attributed to the very different doping dependent broadening behaviors. The results reveal the possibility of GaN based THz-QCL lasing at room temperature.

The spin-polarized tunnel current and the spin-transfer torque in triple-barrier junctions with external electrodes and quantum wells made of ferromagnetic materials

The electron transport in triple-barrier junctions consisting of external ferromagnetic layers and three non-magnetic barriers separated by two inner ferromagnetic layers is analysed. It is assumed that the magnetic moments in the external electrodes are oriented in the same direction, whereas the magnetic moments of the inner layers can form any angle with this direction. The possibility of the gigantic enhancement of the normal torque component has been found in the junctions with a non-colinear orientation of magnetic moments in the inner layers. This enhancement may be much larger than in the double junctions. The current and the in-plane torque are most amplified when the quantum-well states in both inner layers have similar energies and can form the complex resonant states located slightly below the Fermi level. When the tunnelling by these complex resonant states occurs, the torque is greatest in the magnetic configuration other than that where the current is maximally enhanced. Additionally, the angular dependence of the torque differs significantly from that observed in single tunnel junctions.

Moiré-Pattern Modulated Electronic Structures of GaSe/HOPG Heterostructure.

Conventional two-dimensional electron gas (2DEG) typically occurs at the interface of semiconductor heterostructures and noble metal surfaces, but it is scarcely observed in individual 2D semiconductors. In this study, few-layer gallium selenide (GaSe) grown on highly ordered pyrolytic graphite (HOPG) is demonstrated using scanning tunneling microscopy and spectroscopy (STM/STS), revealing that the coexistence of quantum well states (QWS) and 2DEG. The QWS are located in the valence bands and exhibit a peak feature, with the number of quantum wells being equal to the number of atomic layers. Meanwhile, the 2DEG is located in the conduction bands and exhibits a standing-wave feature. Additionally, monolayer GaSe/HOPG heterostructures with different stacking angles (0°, 33°, 8°) form distinct moiré patterns that arise from lattice mismatch and angular rotation between adjacent atomic layers in 2D materials, which effectively modulate the electron effective mass, charge redistribution, and band gap of GaSe. Overall, this work reveals a paradigm of band engineering based on layer numbers and moiré patterns that can modulate the electronic properties of 2D materials.

Quantization condition of strongly correlated electrons in oxide nanostructures

Some oxide nanostructures have recently been shown to host quantum well (QW) states that are promising for quantum device applications by designing the wave function of their strongly correlated electrons. However, it is unclear why QW states only appear in certain oxides, limiting the possibilities of wave-function engineering. Here, we demonstrate that the electron mean free path λ is one of the essential conditions to form standing waves of strongly correlated electrons in oxide nanostructures. We have investigated the QW states formed in SrTi1–xVxO3 (STVO) ultrathin films using in situ angle-resolved photoemission spectroscopy. The controllability of λ in STVO while maintaining an atomically flat surface and chemically abrupt interface enables us to examine the evolution of QW states with varying λ. A detailed analysis reveals that the intensity of the QW states is almost linearly correlated to λ and may disappear at the Ioffe–Regel criterion, corresponding to the nearest neighbor inter-vanadium distance in STVO. Our findings provide design guidelines for creating and controlling novel quantum phenomena in oxide nanostructures.

Injection current and temperature dependence of the optoelectronic properties for electrical-stressed InGaN based micro-LEDs

InGaN-based micro-LEDs are promising for many applications, including visible light communication (VLC), micro-display, etc. However, to realize the above full potential, it is important to understand the degradation behaviors and physical mechanisms of micro-LEDs during operation. Here, the optoelectronic properties of InGaN-based blue micro-LEDs were investigated over a wide range of injection currents (1-100 mA) and temperatures (5-300 K) before and after stress. The results show that the optical power of the micro-LED degrades after stress, especially at lower current density, indicating that the Shockley-Read-Hall (SRH) nonradiative recombination increased for the stressed device. In addition, the slopes of log L-log I curves changes from 1.0 to 2.1 at low current density, and the ideality factor extracted from the I-V curves change from 1.9 to 3.4 after current stress, indicating there is an increase of the defects in the active layer after stress. The activation energy of defects evaluated from the temperature-dependent electroluminescence (EL) spectra is about 200 meV, which could be related to the N-vacancy related defects. Besides, the peak wavelength, peak energy and the full width at half maximum of the injection current- and temperature-dependent EL spectra were discussed. The electron-hole pair combines in the form of SRH nonradiative recombination, causing some carriers to redistribute and a state-filling effect in higher-energy states in multiple quantum wells (MQWs), resulting in the appearance of the shorter-wavelength luminescence in the EL spectra. These findings can help to further understand the degradation mechanisms of InGaN micro-LEDs operated under high current density.

Quantum-well states at the surface of a heavy-fermion superconductor

Two-dimensional electronic states at surfaces are often observed in simple wide-band metals such as Cu or Ag (refs. 1–4). Confinement by closed geometries at the nanometre scale, such as surface terraces, leads to quantized energy levels formed from the surface band, in stark contrast to the continuous energy dependence of bulk electron bands2,5–10. Their energy-level separation is typically hundreds of meV (refs. 3,6,11). In a distinct class of materials, strong electronic correlations lead to so-called heavy fermions with a strongly reduced bandwidth and exotic bulk ground states12,13. Quantum-well states in two-dimensional heavy fermions (2DHFs) remain, however, notoriously difficult to observe because of their tiny energy separation. Here we use millikelvin scanning tunnelling microscopy (STM) to study atomically flat terraces on U-terminated surfaces of the heavy-fermion superconductor URu2Si2, which exhibits a mysterious hidden-order (HO) state below 17.5 K (ref. 14). We observe 2DHFs made of 5f electrons with an effective mass 17 times the free electron mass. The 2DHFs form quantized states separated by a fraction of a meV and their level width is set by the interaction with correlated bulk states. Edge states on steps between terraces appear along one of the two in-plane directions, suggesting electronic symmetry breaking at the surface. Our results propose a new route to realize quantum-well states in strongly correlated quantum materials and to explore how these connect to the electronic environment.

A Quantum Cascade Detector With Enhanced Extraction Efficiency Utilizing Localized Electric Field

An InGaAs/AlInAs quantum cascade detector (QCD) with localized electric field is proposed to produce a 40% higher extraction efficiency of photon-induced electrons when comparing with a conventional QCD. The localized electric field introduced by modulation doping extends the degrees of freedom in the regulation of electronic states in quantum wells, which has significant influence on the transport of electrons. The electric field can enhance the extraction efficiency by enlarging the transition matrix element related to the extraction process. The proposed mid-IR QCD demonstrates a peak responsivity of 156.9 mA/W and a Johnson noise limited detectivity of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.9\times 10 ^{11}$ </tex-math></inline-formula> Jones at 80K.

Strain-induced exciton localized states in quantum well

We use AlAs sacrificial layer etching technology to peel a film of In0.15Ga0.85As/GaAs single quantum wells (QWs) from a GaAs substrate and transfer the film to a Si wafer covered with randomly distributed Au nanoparticles. In this way, local strains are introduced into the QW film, which leads to the formation of the exciton confinement potential. From them, we have observed very sharp emission lines in photoluminescence (PL) spectra and local PL bright spots in space. The PL is proved to be from localized exciton emission in a potential energy funnel. We can also introduce the localized strain in the QW film using a micropillar array of an electron beam resist and observe similar narrow emission lines. The method to achieve localized exciton states is a controllable way and it will further expand the application of traditional semiconductor QWs in optical quantum devices.