Hole Confinement Research Articles

Ab initio study of the effect of 2D layer rippling on the electronic properties of 2D/H-terminated diamond (100) heterostructures

We report an ab initio study of the effect of rippling on the structural and electronic properties of the hexagonal Boron Nitride (hBN) and graphene two-dimensional (2D) layers and heterostructures created by placing these layers on the Hydrogen-terminated (H-) diamond (100) surface. Surprisingly, in graphene, rippling does not open a band gap at the Dirac point but does cause the Dirac cone to be shifted and distorted. For the 2D/H-diamond (100) heterostructures, a combined sampling and a clustering approach were used to find the most favorable alignment of the 2D layers. Heterostructures with rippled layers were found to be the most stable. A larger charge transfer was observed in the heterostructures with rippled hBN (graphene) than their planner counterparts. Band offset analysis indicates a Type-II band alignment for both the wavy and planar heterostructures, with the corrugated structure having stronger hole confinement due to the larger valence band offset between the hBN layer and the H-diamond (100) surface.Graphic abstract

Multiple-quantum-well-induced unipolar carrier transport multiplication in AlGaN solar-blind ultraviolet photodiode

AlGaN solar-blind ultraviolet (SBUV) detectors have potential application in fire monitoring, corona discharge monitoring, or biological imaging. With the promotion of application requirements, there is an urgent demand for developing a high-performance vertical detector that can work at low bias or even zero bias. In this work, we have introduced a photoconductive gain mechanism into a vertical AlGaN SBUV detector and successfully realized it in a p - i - n photodiode via inserting a multiple-quantum-well (MQW) into the depletion region. The MQW plays the role of trapping holes and increasing carrier lifetime due to its strong hole confinement effect and quantum confinement Stark effect. Hence, the electrons can go through the detector multiple times, inducing unipolar carrier transport multiplication. Experimentally, an AlGaN SBUV detector with a zero-bias peak responsivity of about 0.425 A/W at 233 nm is achieved, corresponding to an external quantum efficiency of 226%, indicating the existence of internal current gain. When compared with the device without MQW structure, the gain is estimated to be about 10 3 in magnitude. The investigation provides an alternative and effective approach to obtain high current gain in vertical AlGaN SBUV detectors at zero bias.

Two-dimensional hole gas in organic semiconductors.

A highly conductive metallic gas that is quantum mechanically confined at a solid-state interface is an ideal platform to explore non-trivial electronic states that are otherwise inaccessible in bulk materials. Although two-dimensional electron gases have been realized in conventional semiconductor interfaces, examples of two-dimensional hole gases, the counterpart to the two-dimensional electron gas, are still limited. Here we report the observation of a two-dimensional hole gas in solution-processed organic semiconductors in conjunction with an electric double layer using ionic liquids. A molecularly flat single crystal of high-mobility organic semiconductors serves as a defect-free interface that facilitates two-dimensional confinement of high-density holes. A remarkably low sheet resistance of 6 kΩ and high hole-gas density of 1014 cm-2 result in a metal-insulator transition at ambient pressure. The measured degenerate holes in the organic semiconductors provide an opportunity to tailor low-dimensional electronic states using molecularly engineered heterointerfaces.

First-principles study on improvement of two-dimensional hole gas concentration and confinement in AlN/GaN superlattices

Using first-principles calculations based on density functional theory, we have systematically studied the influence of in-plane lattice constant and thickness of slabs on the concentration and distribution of two-dimensional hole gas (2DHG) in AlN/GaN superlattices. We show that the increase of in-plane lattice constant would increase the concentration of 2DHG at interfaces and decrease the valence band offset, which may lead to a leak of current. Increasing the thickness of AlN and/or decreasing the thickness of GaN would remarkably strengthen the internal field in GaN layer, resulting in better confinement of 2DHG at AlN/GaN interfaces. Therefore, a moderate larger in-plane lattice constant and thicker AlN layer could improve the concentration and confinement of 2DHG at AlN/GaN interfaces. Our study could serve as a guide to control the properties of 2DHG at III-nitride interfaces and help to optimize the performance of p-type nitride-based devices.

Nanocrystal Quantum Dots: From Discovery to Modern Development.

This review traces nanocrystal quantum dot (QD) research from the early discoveries to the present day and into the future. We describe the extensive body of theoretical and experimental knowledge that comprises the modern science of QDs. Indeed, the spatial confinement of electrons, holes, and excitons in nanocrystals, coupled with the ability of modern chemical synthesis to make complex designed structures, is today enabling multiple applications of QD size-tunable electronic and optical properties.

Hole transport enhancement by thickness- and composition-grading of electron blocking layer

We have numerically examined the advantages of thickness- and composition-grading of the electron blocking layer (EBL) in InGaN multiquantum well light-emitting diodes. We have enhanced the hole confinement inside the active region, which is critical in GaN-based devices. Low hole injection is more severe when conventional wide bandgap AlGaN EBL is inserted between the last GaN quantum barrier and the p-GaN layer. The results obtained show reduced valence band offset leading to improved hole injection and enhanced device performance.

Photoelectrochemical investigation of charge injection efficiency for quantum dot light-emitting diode

When we applied colloidal quantum dots (QDs) for quantum dot light emitting diodes, it was well known that shell thickness played an important role in core protection, confinement of electrons and holes, and charge injection efficiency. However, although the shell thickness dependence of electroluminescence properties was reported, carrier injection efficiency has not been discussed in detail. In this paper, we investigated the effect of shell thickness on the carrier injection efficiency that was evaluated by photoelectrochemical measurements. By comparing the product of internal quantum yield of photoluminescence and the evaluated carrier injection efficiency with external quantum efficiency (EQE) for QDs with various shell thicknesses, we found that the optimal shell thickness for increasing EQE is determined by the balance between protection of QD's surface and carrier injection efficiency.

“Can quantum dot in form of molecular ground be implied in IETS to measure the voltage effectively by sudden potential rise due to phonon assisted electron tunneling as Klein paradox?”

Abstract Sudden break in the passage of electron beam things conducting barrier shall either enhance or decrease electron beam with provided effect on spectral band of unknown metal, j/?dv –V at d2/dv2 plot gives characteristic of tunnel junction. At liquid helium temperature qv = hὐ implication for JV curve is observed at bias V where v denotes the frequency of molecular compounds present in the barrier, In case barrier emitting phonon interacts with the lattice of implied nano particles like Ga, As Ge Se coupled barrier in molecular state can be observed a specific band of Raman spectra and infra red thermally nactivated long wave spectra as IETS under Josephson effect. Single quantum dot in ultraclean carbonnanotube can be annexed with novel double quantum dot as result of klein paradox. At relatvistic QFT. In this context single spin1/2 can be confined with the hole emplacement and confinement as second double dot.

Electronic properties of type-II $$\hbox {GaAs}_{1-x} \hbox {Sb}_{x}$$/GaAs quantum rings for applications in intermediate band solar cells

We present a theoretical analysis of the electronic properties of type-II $$\hbox {GaAs}_{1-x} \hbox {Sb}_{x}$$ /GaAs quantum rings (QRs), from the perspective of applications in intermediate band solar cells (IBSCs). We outline the analytical solution of Schrodinger’s equation for a cylindrical QR of infinite potential depth, and describe the evolution of the QR ground state with QR morphology. Having used this analytical model to elucidate general aspects of the electronic properties of QRs, we undertake multi-band k·p calculations—including strain and piezoelectric effects—for realistic $$\hbox {GaAs}_{1-x} \hbox {Sb}_{x}$$ /GaAs QRs. Our k·p calculations confirm that the large type-II band offsets in $$\hbox {GaAs}_{1-x} \hbox {Sb}_{x}$$ /GaAs QRs provide strong confinement of holes, and further indicate the presence of resonant (quasi-bound) electron states which localise in the centre of the QR. From the perspective of IBSC design the calculated electronic properties demonstrate several benefits, including (i) large hole ionisation energies, mitigating thermionic emission from the intermediate band, and (ii) electron-hole spatial overlaps exceeding those in conventional $$\hbox {GaAs}_{1-x} \hbox {Sb}_{x}$$ /GaAs QDs, with the potential to engineer these overlaps via the QR morphology so as to manage the trade-off between optical absorption and radiative recombination. Overall, our analysis highlights the flexibility offered by the QR geometry from the perspective of band structure engineering, and identifies specific combinations of QR alloy composition and morphology which offer optimised sub-band gap energies for QR-based IBSCs.

Hole-Trapping-Induced Stabilization of Ni4 + in SrNiO3 /LaFeO3 Superlattices.

Creating new functionality in materials containing transition metals is predicated on the ability to control the associated charge states. For a given transition metal, there is an upper limit on valence that is not exceeded under normal conditions. Here, it is demonstrated that this limit of 3+ for Ni and Fe can be exceeded via synthesis of (SrNiO3 )m /(LaFeO3 )n superlattices by tuning n and m. The Goldschmidt tolerance constraints are lifted, and SrNi4+ O3 with holes on adjacent O anions is stabilized as a perovskite at the single-unit-cell level (m= 1). Holding m= 1, spectroscopy reveals that the n= 1 superlattice contains Ni3+ and Fe4+ , whereas Ni4+ and Fe3+ are observed in the n= 5 superlattice. It is revealed that the B-site cation valences can be tuned by controlling the magnitude of the FeO6 octahedral rotations, which, in turn, determine the energy balance between Ni3+ /Fe4+ and Ni4+ /Fe3+ , thus controlling emergent electrical properties such as the band alignment and resulting hole confinement. This approach can be extended to other systems for synthesizing novel, metastable layered structures with new functionalities.

Temperature dependence of hole transport properties through physically defined silicon quantum dots

For future integration of a large number of qubits and complementary metal-oxide-semiconductor (CMOS) controllers, higher operation temperature of qubits is strongly desired. In this work, we fabricate p-channel silicon quantum dot (Si QD) devices on silicon-on-insulator for strong confinement of holes and investigate the temperature dependence of Coulomb oscillations and Coulomb diamonds. The physically defined Si QDs show clear Coulomb diamonds at temperatures up to 25 K, much higher than for gate defined QDs. To verify the temperature dependence of Coulomb diamonds, we carry out simulations and find good agreement with the experiment. The results suggest a possibility for realizing quantum computing chips with qubits integrated with CMOS electronics operating at higher temperature in the future.

Determination of the interface band alignment of NiO/4H-SiC heterojunction for photodetector application

The band alignment of NiO/4H-SiC heterojunction is firstly investigated by X-ray photoelectron spectroscopy (XPS) and transmission spectra measurements. The valence (ΔEV) and conduction band offset (ΔEC) are determined as 2.5±0.01 and 3.0±0.01eV, respectively, which is the result of considering the effect of interfacial band bending. Furthermore, the band alignment is evaluated to be staggered (Type-II), which gives rise to the confinement of electrons and holes in the NiO layer and 4H-SiC, respectively. Hence, the large ΔEC (ΔEV) values and the type-II band alignment can contribute to the separation of photogenerated carriers for optoelectronic applications.

Effectively Confining the Lateral Current Within the Aperture for GaN Based VCSELs by Using a Reverse Biased NP Junction

GaN-based vertical cavity surface emitting laser (VCSEL) features the quasi-vertical architecture, and thus a poor lateral current can be encountered, which can significantly reduce the net modal gain and decrease the lasing power. To prevent the holes from lateral leaking outside of the aperture region, we propose GaN based VCSELs with an NP-GaN structure below the buried insulator layer. The reverse biased NP-GaN junction can produce energy barrier that can better suppress the lateral hole leakage outside of the aperture, therefore the better stimulated radiative rate and device lasing power are obtained. Moreover, we also conduct parametric study regarding the impact of different NP-GaN junction designs with various doping concentrations and architectural sizes for the p-GaN layer on the lateral hole confinement capability.

Neutral excitation density-functional theory: an efficient and variational first-principles method for simulating neutral excitations in molecules

We introduce neutral excitation density-functional theory (XDFT), a computationally light, generally applicable, first-principles technique for calculating neutral electronic excitations. The concept is to generalise constrained density functional theory to free it from any assumptions about the spatial confinement of electrons and holes, but to maintain all the advantages of a variational method. The task of calculating the lowest excited state of a given symmetry is thereby simplified to one of performing a simple, low-cost sequence of coupled DFT calculations. We demonstrate the efficacy of the method by calculating the lowest single-particle singlet and triplet excitation energies in the well-known Thiel molecular test set, with results which are in good agreement with linear-response time-dependent density functional theory (LR-TDDFT). Furthermore, we show that XDFT can successfully capture two-electron excitations, in principle, offering a flexible approach to target specific effects beyond state-of-the-art adiabatic-kernel LR-TDDFT. Overall the method makes optical gaps and electron-hole binding energies readily accessible at a computational cost and scaling comparable to that of standard density functional theory. Owing to its multiple qualities beneficial to high-throughput studies where the optical gap is of particular interest; namely broad applicability, low computational demand, and ease of implementation and automation, XDFT presents as a viable candidate for research within materials discovery and informatics frameworks.

High internal quantum efficiency of green GaN-based light-emitting diodes by thickness-graded last well/last barrier and composition-graded electron blocking layer

To enhance the performance of green light-emitting diodes (LEDs), numerical analysis of structural engineering of conventional LED has been presented. Our proposed structure with composition-graded electron blocking layer as well as thickness-graded last quantum barrier and last quantum well shows improved radiative recombination rate, lower efficiency droop and higher light output power at high current density in comparison to other devices. The observed improvements are because of better electron and hole confinement as well as improved hole transportation inside the active region.

Enhancing the lateral current injection by modulating the doping type in the p-type hole injection layer for InGaN/GaN vertical cavity surface emitting lasers.

In this report, we propose GaN-based vertical cavity surface emitting lasers with a p-GaN/n-GaN/p-GaN (PNP-GaN) structured current spreading layer. The PNP-GaN current spreading layer can generate the energy band barrier in the valence band because of the modulated doping type, which is able to favor the current spreading into the aperture. By using the PNP-GaN current spreading layer, the thickness for the optically absorptive ITO current spreading layer can be reduced to decrease internal loss and then enhance the lasing power. Furthermore, we investigate the impact of the doping concentration, the thickness and the position for the inserted n-GaN layer on the lateral hole confinement capability, the lasing power, and the optimization strategy. Our investigations also report that the optimized PNP-GaN structure will suppress the thermal droop of the lasing power for our proposed VCSELs.

On the origin for the hole confinement into apertures for GaN-based VCSELs with buried dielectric insulators.

A better lateral current confinement is essentially important for GaN-based vertical-cavity-surface-emitting lasers (VCSELs) to achieve lasing condition. Therefore, a buried insulator aperture is adopted. However, according to our results, we find that the current cannot be effectively laterally confined if the insulator layer is not properly selected, and this is because of the unique feature for GaN-based VCSELs grown on insulating substrates with both p-electrode and n-electrode on the same side. Our results indicate that the origin for the current confinement arises from lateral energy band bending in the p-GaN layer rather than the electrical resistivity for the buried insulator. The lateral energy band in the p-GaN layer can be more flattened by using a buried insulator with a properly larger dielectric constant. Thus, less bias can be consumed by the buried insulator, enabling better lateral current confinement. On the other hand, the bias consumption by the buried insulator is also affected by the insulator thickness, and we propose to properly decrease the insulator layer thickness for reducing the bias consumption therein and achieving better lateral current confinement. The improved lateral current confinement will correspondingly enhance the lasing power. Thanks to the enhanced lateral current confinement, the 3dB frequency will also be increased if proper buried insulators are adopted.

Core/Shell Quantum Dots Solar Cells

AbstractSemiconductor nanocrystals, the so‐called quantum dots (QDs), exhibit versatile optical and electrical properties. However, QDs possess high density of surface defects/traps due to the high surface‐to‐volume ratio, which act as nonradiative carrier recombination centers within the QDs, thereby deteriorating the overall solar cell performance. The surface passivation of QDs through the growth of an outer shell of different materials/compositions called “core/shell QDs” has proven to be an effective approach to reduce the surface defects and confinement potential, which can enable the broadening of the absorption spectrum, accelerate the carrier transfer, and reduce exciton recombination loss. Here, the recent research developments in the tailoring of the structure of core/shell QDs to tune exciton dynamics so as to improve solar cell performance are summarized. The role of band alignment of core and shell materials, core size, shell thickness/compositions, and interface engineering of core/thick shell called “giant” QDs on electron–hole spatial separation, carrier transport, and confinement potential, before and after grafting on the carrier scavengers (semiconductor/electrolyte), is described. Then, the solar cell performance based on core/shell QDs is introduced. Finally, an outlook for the rational design of core/shell QDs is provided, which can further promote the development of high‐efficiency and stable QD sensitized solar cells.

From "weak" to "strong" hole confinement in a Mott insulator

We study the problem of a single hole in an Ising antiferromagnet and, using the magnon expansion and analytical methods, determine the expansion coefficients of its wave function in the magnon basis. In the 1D case, the hole is “weakly” confined in a potential well and the magnon coefficients decay exponentially in the absence of a string potential. This behavior is in sharp contrast to the 2D square lattice where the hole is “strongly” confined by a string potential and the magnon coefficients decay superexponentially. The latter is identified here to be a fingerprint of the strings in doped antiferromagnets that can be recognized in the numerical or cold atom simulations of the 2D doped Hubbard model. Finally, we attribute the differences between the 1D and 2D cases to the magnon-magnon interactions being crucially important in a 1D spin system.

Type-II AlInN/ZnGeN2 quantum wells for ultraviolet laser diodes

We propose a type-II AlInN/ZnGeN2 quantum well (QW) structure serving as the active region for ultraviolet (UV) laser diodes. A remarkably low threshold current density can be achieved using the type-II AlInN/ZnGeN2 QW structure, providing a pathway for the realization of electrically-driven nitride-based semiconductor UV laser diodes. ZnGeN2 has both a very similar lattice constant and bandgap to GaN. Its large band offsets with GaN enable the potential of serving as a hole confinement layer to increase the electron-hole wavefunction overlap in the active region. In this study, we investigate the spontaneous emission and gain properties of type-II AlInN/ZnGeN2 QWs with different ZnGeN2 layer thicknesses. Our findings show that the use of ZnGeN2 layers in the active region provides a significant improvement in hole carrier confinement, which results in ∼5 times enhancement of the electron-hole wave function overlap. Such an enhancement provides the ability to achieve a significant increase (∼6 times) in the spontaneous emission rate and material gain, along with a remarkable reduction in threshold carrier density compared to the conventional AlGaN-based QW design, which is essential for practical UV laser diodes.