Strong Built-in Electric Field Research Articles

DONOR BOUND EXCITONS CONFINED IN WURTZITE InGaN/GaN QUANTUM DOT NANOWIRE HETEROSTRUCTURES

Within the framework of the effective-mass approximation, the donor bound exciton states and interband optical transitions in InGaN / GaN strained quantum dot (QD) nanowire heterostructures (NWHETs) are investigated using a variational method, in which the built-in electric field (BEF) effect due to the spontaneous and piezoelectric polarizations and the three-dimensional (3D) confinement of the electron and hole in the QD are considered. Our results show that the position of the ionized donor, the strong BEF, the In-composition and the QD structural parameters have a significant influence on the donor bound exciton binding energy, the electron interband optical transitions and the exciton oscillator strength. The donor bound exciton binding energy increases obviously if the donor position changes from the left-interface to the right-interface. The BEF reduces the bound exciton binding energy and the exciton oscillator strength. The donor bound exciton binding energy increases, if the In-fraction increases. The emission wavelength monotonically increases if the QD height and radius increase.

On the donor states in double InxGa1 − xN/InyGa1 − yN/GaN staggered quantum wells

We have calculated the binding energies of the donor states, 1s and 2p±, with respect to the lowest sub-band energy in a double quantum well composed of wurtzite InGaN staggered quantum wells with GaN barriers. All the energies and the wavefunctions were calculated by applying the variational methods. We have found that the binding energies of donors placed in the right quantum well are larger and independent of the middle barrier width of up to 40 Å. This is because of the strong built-in electric field which brings more confinement to the donor wavefunctions in the right staggered quantum well. The binding energies are found to be strong functions of the donor position in the double quantum well system which is the consequence of the large asymmetry introduced by the built-in electric field.

In Situ Integration of Squaraine-Nanowire-Array-Based Schottky-Type Photodetectors with Enhanced Switching Performance

Organic nanostructure-based photodetectors are important building blocks for future high-performance, low-cost, flexible nano-optoelectronic devices. However, device integration remains a large challenge, and the structure-dependent performance of the device has been seldom studied. Here, we report the in situ integration of 2,4-bis[4-(N,N-dimethylamino)phenyl]squaraine (SQ)-nanowire (NW)-array-based photodetectors by growing the organic NW arrays on prefabricated electrodes through an evaporation-induced self-assembly process. In contrast with ohmic-contact devices, asymmetric electrode pairs of Au-Ti were utilized to achieve the construction of Schottky-type photodetectors on the basis of organic NW arrays. Significantly, the Schottky-type photodetectors exhibited a significantly enhanced performance as compared to the ohmic-type devices in terms of their higher photosensitivity and switching speed. The presence of a strong built-in electric field at the junction interface, which greatly facilitated the separation/transportation of photogenerated electron-hole pairs, was suggested to be responsible for the superior performance of the Schottky-type photodetectors. More importantly, the organic NW-array-based devices also showed a higher sensitivity and reproducibility than that of the single NW-based devices, and they were capable of low-light detection. The investigation of the photodetector circuitry also disclosed a very low pixel-to-pixel variation owing to the average effect of the NW-array-based devices. It is expected that organic NW-array-based Schottky-type photodetectors will have important applications in future organic nano-optoelectronic devices.

Strongly Enhanced THz Emission caused by Localized Surface Charges in Semiconducting Germanium Nanowires

A principal cause of THz emission in semiconductor nanostructures is deeply involved with geometry, which stimulates the utilization of indirect bandgap semiconductors for THz applications. To date, applications for optoelectronic devices, such as emitters and detectors, using THz radiation have focused only on direct bandgap materials. This paper reports the first observation of strongly enhanced THz emission from Germanium nanowires (Ge NWs). The origin of THz generation from Ge NWs can be interpreted using two terms: high photoexcited electron-hole carriers (Δn) and strong built-in electric field (Eb) at the wire surface based on the relation . The first is related to the extensive surface area needed to trigger an irradiated photon due to high aspect ratio. The second corresponds to the variation of Fermi-level determined by confined surface charges. Moreover, the carrier dynamics of optically excited electrons and holes give rise to phonon emission according to the THz region.

Internal field induced exciton binding energy and the optical transition in a strained Mg based II–VI quantum well

Binding energy of an exciton in a wurtzite ZnO/Zn1−xMgxO strained quantum well is investigated theoretically in which the strong built-in electric field due to the spontaneous and piezoelectric polarizations is included. Numerical calculations are performed using variational procedure within the single band effective mass approximation by varying the Mg composition in the barrier. The exciton oscillator strength and the exciton lifetime for radiative recombination as functions of well width and Mg content have been computed. The internal field induced interband emission energy of strained ZnO/Zn1−xMgxO well is investigated with the various structural parameters. The total optical absorption coefficients and the changes of refractive index as a function of normalized photon energy in the presence of built-in internal field are analyzed. The result shows that the strong built-in electric field has influence on the oscillator strength and the recombination life time of the exciton. The optical absorption coefficients and the refractive index changes strongly depend on Mg composition. The occurred blue shift of the resonant peak due to the incorporation of Mg ions will give the information about the variation of two energy levels in the quantum well.

Size-dependent carrier dynamics and activation energy in CdTe/ZnTe quantum dots on Si substrates

Abstract

Magnetic field induced non-linear optical properties in a strained wurtzite GaN/AlxGa1−xN quantum dot: Effect of internal fields

Effect of magnetic field on exciton binding energy is investigated in a strained wurtzite GaN/Al0.2Ga0.8N quantum dot in which the strong built-in electric field due to the spontaneous and piezoelectric polarizations is included. Numerical calculations are performed using variational procedure within the single band effective mass approximation by the magnetic field strength. The magnetic field induced interband emission energy of a strained GaN/AlGaN quantum dot is investigated with and without the effect of internal field. The exciton oscillator strength and the exciton lifetime for radiative recombination as a function of dot radius with the effect of magnetic field strength are computed. The total optical absorption coefficients and the changes of refractive index as a function of normalized photon energy in the presence of built-in internal field are analyzed in the presence of magnetic field. The optical absorption coefficients and the refractive index changes strongly depend on the incident optical intensity and the magnetic field. The occurred blue shift of the resonant peak due to the magnetic field will give the information about the variation of two energy levels in the quantum dot. The optical absorption coefficients and the refractive index changes are strongly dependent on the incident optical intensity, the internal field and the magnetic field.

Active Layer Spin Coating Speed Dependence of Inverted Organic Solar Cell Based on Eosin-Y-Coated ZnO Nanorod Arrays

The active layer spin coating speed dependence of the performance of inverted organic solar cells (OSCs) based on Eosin-Y-coated ZnOnanorods has been investigated. An active layer consisted of poly(2-methoxy-5-(2'-ethyl)-hexyloxy-p-phenylenevinylene) (MEH-PPV) as donor and phenyl-c61-butyric acid methyl ester (PCBM) as acceptor was employed, whereas ZnO nanorods were utilized as electron transporting layer. The active layer was deposited on top of Eosin-Y-coated ZnO nanorods with various spin coating speeds (1000-4000 rpm). Inverted OSCs with a structure of FTO/Eosin-Y-coated ZnO nanorods/MEH-PPV:PCBM /Ag were characterized through the current density-voltage (J-V) measurement under illumination intensity of 100 mW/cm2. Based on the investigation, the short circuit current density (Jsc) and the power conversion efficiency (PCE) enhanced significantly, where as fill factor slightly increased with spin coating speed. The two-diode equivalent model was found to fit the experimental J-V curves very well. The optimum PCE of 1.18 ± 0.07% was achieved at the highest spin coating speed of 4000 rpm, as a result of the decrement of diffusion current density (Jdiff), recombination current density (Jrec), and ideality factor, thus further confirms the strong built-in electric field in thinner photoactive layer.

Nonlocal optical properties in InGaN/GaN strained quantum wells with a strong built-in electric field

Based on the microscopic nonlocal optical response theory, the linear and nonlinear intersubband optical absorption properties in InyGa1−yN/GaN strained single quantum wells (SQWs) with a strong built-in electric field are investigated. The numerical results show that the optical absorption spectrum has an obvious blueshift originating from the nonlocal optical responses. The dependence of the blueshift on the SQW width and the mixing fraction y is clarified. In addition, the influence of the SQW structural parameters, the incident light intensity, and the incident angle on the optical absorption properties is demonstrated. These properties are desirable for controlling the nonlocal effect and optical absorption loss in optical device application.

First-Principles Study on New Photocatalytic Materials Graphdiyne-TiO2

We study mechanism for combinet graphdiyne and graphene on TiO2 which can improve the photocatalytic performance of TiO2 by density functional theory with dispersion corrections (DFT-D) methods. Because of the different lattice parameters of graphdiyne, graphene and TiO2(101), we used (1 x 1) graphdiyne supercell with (1 x 4) TiO2(101) supercell to built the graphdiyne-TiO2(101) composite, and (2 x 6) graphdiyne supercell with (1 x 5) TiO2(101) supercell to built the graphene-TiO2(101) composite. In detail, as the strong adsorption between carbon atom and the TiO2 support, numbers of new high electron delocalization C-O covalent bonds were formed by O atom in TiO2 surface and atop C atom in graphdiyne. While for the graphene-TiO2(101) composite, the equilibrium distances between graphene and TiO2 are so large that even the nearest C atoms were far as 2.869 A to the TiO2 planes, indicating the weak interaction between graphene and TiO2 surface. The electron density, electron density difference were analyzed, the result shown that the graphdiyne combined with TiO2(101) surface is beneficial to the charge transfer. The Mulliken charge population shows that the graphdiyne or graphene surface has a positive Mulliken charge, forming large opposite interface dipole at the interface, leading to a strong built-in electric field throughout the superlattice, which can resist the possibility of electron-hole recombination. Since the magnitude of charge accumulation in graphdiyne is larger than graphene surface, stronger electrons capture ability of graphdiyne TiO2(101)composite could be expected compared to graphene-TiO2(101) composite. The electronic band structure shown that in the graphdiyne-TiO2(101) composite, numbers of isolated energy levels were localized in the band gap which can reduce the electronic excitation energy from the isolated energy levels to the conductive band. The graphene-TiO2(101) composite had no isolated energy level appeared in the band gap. The graphdiyne-TiO2(101) composite has a lower valence band position than the graphene-TiO2(101) composite ones, means compared with graphene composite, graphdiyne composite always shows higher oxidation ability, which induce a higher photocatalytic performance.

Influence of Mg Composition on Optical Properties of Exciton Confinement in Strained Wurtzite ZnO/MgxZn1−xO Cylindrical Quantum Dots

Based on the effective-mass approximation and variational approach, excitonic optical properties are investigated theoretically in strained wurtzite (WZ) ZnO/MgxZn1−xO cylindrical quantum dots (QDs) for four different Mg compositions: x = 0.08, 0.14, 0.25, and 0.33, with considering a three-dimensional carrier confinement in QDs and a strong built-in electric field effect due to the piezoelectricity and spontaneous polarization. The ground-state exciton binding energy, the interband emission wavelength, and the radiative lifetime as functions of the QD structural parameters (height and radius) are calculated in detail. The computations are performed in the case of finite band offset. Numerical results elucidate that Mg composition has a significant influence on the exciton states and optical properties of ZnO/MgxZn1−xO QDs. The ground-state exciton binding energy increases with increasing Mg composition and the increment tendency is more prominent for small height QDs. As Mg composition increases, the interband emission wavelength has a blue-shift if the dot height L < 3.5 nm, but the interband emission wavelength has a red-shift when L > 3.5 nm. Furthermore, the radiative lifetime increases rapidly with increasing Mg composition if the dot height L > 3 nm and the increment tendency is more prominent for large height QDs. The physical reason has been analyzed in depth.

Binding energies and oscillator strengths of impurity states in wurtzite InGaN/GaN staggered quantum wells

Using the variational methods, we have calculated the binding energies of the lowest donor states, 1s and 2p±, in wurtzite InGaN/GaN staggered quantum wells. The binding energies in narrow wells are larger in magnitude than the values in bulk GaN due to the quantum confinement effects. However, the energies decrease sharply in wider wells because of the weakening confinement due to the strong built-in electric field inside the well. The binding energies of donors placed at the opposite edges of the well are quite different as the built-in electric field forms an asymmetric, triangular potential inside the well. The oscillator strength of the possible transitions between the donor states is then computed by modelling them as the states of a two-level atom. A magnetic field applied along the growth direction splits up the degenerate 2p± states. The amount of splitting in the quantum well is found to be small possibly due to the heavy electron effective mass inside the well. The oscillator strength of the transition between the donor states becomes greater with the increasing magnetic field.

Influence of pressure on exciton states and interband optical transitions in wurtzite InGaN/GaN coupled quantum dot nanowire heterostructures with polarization and dielectric mismatch

Considering the hydrostatic pressure, the spontaneous and piezoelectric polarization, the dielectric mismatch, and 3D confinement of the electron and hole, the exciton states and interband optical transitions in [0001]-oriented wurtzite InxGa1−xN/GaN strained coupled quantum dot (QD) nanowire heterostructures (NWHETs) have been investigated by using the effective mass approximation, the simplified coherent potential approximation, and a variational approach. Our results show that the hydrostatic pressure, the strong built-in electric field (BEF), and the dielectric mismatch have a significant influence on the exciton states and interband optical transitions. The exciton binding energy increases almost linearly with the hydrostatic pressure for a given QD NWHET. The emission wavelength has a blue-shift (red-shift) if the hydrostatic pressure (QD height or the potential barrier thickness) increases. Our calculations also indicate that the radiative decay time has a quick increase with increasing of the QD height and the barrier thickness. The radiative decay time decreases if the hydrostatic pressure increases. The BEF (dielectric mismatch) dramatically decreases (increases) the exciton binding energy. The physical reason has been analyzed in depth.

Investigation of self-focusing effects in wurtzite InGaN/GaN quantum dots

The third-order nonlinear optical properties in wurtzite InGaN/GaN pyramid and truncated-pyramid quantum dots are studied, and the oscillator strength, third-order nonlinear optical susceptibility and self-focusing effects are analyzed theoretically taken into account the strong built-in electric field effect due to the piezoelectric and spontaneous polarization in nitride materials. The numerical results clearly show that the quantum dot (QD) size of InGaN/GaN have a significant influence on the nonlinear optical properties of wurtzite InGaN/GaN quantum dots. Furthermore, the self-focusing effect increases with decrease in size of QDs.

Allowed and forbidden Raman scattering mechanisms for detection of coherent LO phonon and plasmon-coupled modes in GaAs

Detection mechanisms of coherent phonons in variously doped GaAs are investigated by transient reflectivity method with photoexcitation near the ${E}_{0}$ gap and probe near either the ${E}_{0}$ or ${E}_{1}$ gaps. By varying the probe light polarization angle, the coherent amplitudes of both the LO phonon and the LO phonon-plasmon coupled (LOPC) modes show evidence for an interference between their anisotropic and isotropic dielectric response components. We attribute the anisotropic and isotropic components to the reflectivity modulation by lattice and electronic polarizations via dipole-allowed and dipole-forbidden Raman scattering processes. The forbidden processes are resonantly enhanced at the ${E}_{0}$ and ${E}_{1}$ critical points due to the relaxation of momentum conservation and the strong built-in electric field near the surface. The relative contribution of the anisotropic and isotropic components depends on the modes (LO or LOPC) as well as the probe wavelength (${E}_{0}$ or ${E}_{1}$), because of the different Raman scattering mechanisms involved. Reflectivity measurements with the near-ultraviolet light, which is used to probe the ${E}_{1}$ gap, also enable highly surface sensitive detection of ultrafast LO phonon-plasmon dynamics, which is strongly depth dependent in the depletion layer of $n$-doped GaAs.

Optimization of optical gain in AlxGa1−xN/GaN/AlxGa1−xN strained quantum well laser

In this paper the optical gain in wurtzite AlGaN/GaN quantum well is studied. The effects of temperature, carrier concentration, quantum well width, and barrier width are analyzed theoretically taken into account the strong built-in electric field effect due to the piezoelectric and spontaneous polarization in the nitride materials. The numerical results clearly show that the increasing of carrier concentration, and decreasing of temperature and well widths, the optical gain increases.

Effect of built-in electric field on electron Raman scattering in InGaN/GaN coupled quantum wells

Electron Raman scattering (ERS) in wurtzite In x GaN 1− x /GaN coupled quantum wells (CQWs) is investigated by effective-mass approximation and second-perturbation approach, including a strong built-in electric field (BEF) effect due to the piezoelectricity and spontaneous polarization. The dependence of differential cross-section (DCS) on structural parameters of CQWs is studied. Our results show that the strong BEF gives rise to a remarkable reduction of the DCS, which is around three orders smaller than that of the CQWs without BEF. With the presence of the BEF, the emitted photon energy decreases about 10 times as a consequence of quantum-confined Stark effect.

Exciton states and interband optical transitions in wurtzite InGaN/GaN quantum dot nanowire heterostructures

Within the framework of the effective-mass approximation, the exciton states and interband optical transitions in In x Ga 1− x N/GaN strained quantum dot (QD) nanowire heterostructures are investigated using a variational method, in which the important built-in electric field (BEF) effects, dielectric-constant mismatch and three-dimensional confinement of the electron and hole in In x Ga 1− x N QDs are considered. We find that the strong BEF gives rise to an obvious reduction of the effective band gap of QDs and leads to a remarkable electron-hole spatial separation. The BEF, QD height and radius, and dielectric mismatch effects have a significant influence on exciton binding energy, electron interband optical transitions, and the exciton oscillator strength.

Radiative life time of an exciton confined in a strained GaN/Ga1-xAlxN cylindrical dot: built-in electric field effects

The binding energy of an exciton in a wurtzite GaN/GaAlN strained cylindrical quantum dot is investigated theoretically. The strong built-in electric field due to the spontaneous and piezoelectric polarizations of a GaN/GaAlN quantum dot is included. Numerical calculations are performed using a variational procedure within the single band effective mass approximation. Valence-band anisotropy is included in our theoretical model by using different hole masses in different spatial directions. The exciton oscillator strength and the exciton lifetime for radiative recombination each as a function of dot radius have been computed. The result elucidates that the strong built-in electric field influences the oscillator strength and the recombination life time of the exciton. It is observed that the ground state exciton binding energy and the interband emission energy increase when the cylindrical quantum dot height or radius is decreased, and that the exciton binding energy, the oscillator strength and the radiative lifetime each as a function of structural parameters (height and radius) sensitively depend on the strong built-in electric field. The obtained results are useful for the design of some opto-photoelectronic devices.

Intersubband absorption in strained AlGaN/GaN double quantum wells

The intersubband absorption of the four-energy-level system in strained AlGaN/GaN double quantum wells is calculated by considering the polarization effect and the strain modification on material parameters (e.g., the conduction band offset, the electron effective mass and the static dielectric constant). It is found that the electron wavefunctions mainly locate at the left well and penetrate into the left barrier. The absorption spectrum exhibits multiple peaks contributed by different transitions. The position and height of absorption peaks are not very sensitive to the structural parameters (i.e., composition and thickness) of the central barrier because of the strong built-in electric field. However, the coupling between two wells can be enhanced by strain modulation.