Strong Built-in Electric Field Effect Research Articles

Enhanced CO2-to-methane photoconversion over carbon nitride via interfacial charge kinetics steering

Serious carrier recombination and limited active sites are the main restrictions on the efficiency of CO2 photoconversion over g-C3N4-based photocatalysts. Herein, a unique tubular C3N4-carbon heterostructure was constructed via thermal polycondensation process, where the inherent open porous C3N4 microstructure was rationally exploited as scaffolds to co-assemble with pyridinic N moieties into composites. Beyond the critical role of serving as a carbon source, the EDTA-2Na additive provides a fairly straightforward pathway for the production of surface cyano groups, which presumably results from the dehydration process of amide bonds. The interfacial cyano groups and work function difference induce strong built-in electric field effects, boosting spatial carrier transfer kinetics. Meanwhile, the incorporated reactive N species endow high affinity for CO2 and thereby remarkable enhancement of uptake capacity. The developed hybrid system modulates the CO2 reduction pathway with favoured 8e− selectivity (56%) toward CH4 products, with an evolution rate up to 2.54 μmol·g-1 h−1, compared with trace of the bare C3N4 counterpart. This work constitutes a prospective target for surface and interface design and paves the way for developing high-performance photocatalysts for multi-electron CO2 reduction.

High-performance broadband photoresponse of self-powered Mg2Si/Si photodetectors

Infrared optoelectronic devices are capable of operating in harsh environments with outstanding confidentiality and reliability. Nevertheless, suffering from the large band gap value, most semiconductor materials are difficult to detect infrared light signals. Here, Mg2Si/Si heterojunction photodetectors (PDs), which possess the advantages of low-cost, easy process, environmental friendliness, and compatibility with silicon CMOS technology, have been reported with a broadband spectral response as tested from 532 to 1550 nm under zero-bias. When the incident light wavelength is 808 nm, the Mg2Si/Si photodetector (PD) has a responsivity of 1.04 A W−1 and a specific detectivity of 1.51 × 1012 Jones. Furthermore, we find that the Ag nanoparticles (Ag_NPs) assembled on the Mg2Si layer can greatly improve the performance of the Mg2Si/Si PD. The responsivity and specific detectivity of Mg2Si/Si device with Ag_NPs under 808 nm illumination are 2.55 A W−1 and 2.60 × 1012 Jones, respectively. These excellent photodetection performances can be attributed to the high-quality of our grown Mg2Si material and the strong built-in electric field effect in the heterojunction, which can be further enhanced by the local surface plasmon effect and local electromagnetic field induced by Ag_NPs. Our study would provide significant guidance for the development of new self-powered infrared PDs based on silicon materials.

InGaAs Diodes for Terahertz Sensing-Effect of Molecular Beam Epitaxy Growth Conditions.

InGaAs-based bow-tie diodes for the terahertz (THz) range are found to be well suited for development of compact THz imaging systems. To further optimize design for sensitive and broadband THz detection, one of the major challenges remains: to understand the noise origin, influence of growth conditions and role of defects for device operation. We present a detailed study of photoreflectance, low-frequency noise characteristics and THz sensitivity of InGaAs bow-tie diodes. The diodes are fabricated from InGaAs wafers grown by molecular beam epitaxy (MBE) on semi-insulating InP substrate under different technological conditions. Photoreflectance spectra indicated the presence of strong built-in electric fields reaching up to 49 kV/cm. It was demonstrated that the spectral density of voltage fluctuations at room temperature was found to be proportional to 1/f, while at lower temperatures, 77–200 K, Lorentzian-type spectra dominate due to random telegraph signals caused by individual capture defects. Furthermore, varying bias voltage, we considered optimal conditions for device room temperature operation in the THz range with respect to signal-to-noise ratio. The THz detectors grown with beam equivalent pressure In/Ga ratio equal to 2.04 exhibit the minimal level of the low-frequency noise, while InGaAs layers grown with beam equivalent pressure In/Ga ratio equal to 2.06 are found to be well suited for fabrication of room temperature bow-tie THz detectors enabling sensitivity of 13 V/W and noise equivalent power (NEP) of 200 pW/√Hz at 0.6 THz due to strong built-in electric field effects.

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.

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.

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.

Influence of Mg content on optical properties of exciton confinement in wurtzite ZnO/Mg xZn 1− xO coupled quantum dots

Based on the framework of effective-mass approximation and variational approach, optical properties of exciton are investigated theoretically in ZnO/Mg x Zn 1− x O vertically coupled quantum dots (QDs), with considering the three-dimensional confinement of electron and hole pair and the strong built-in electric field effects. The exciton binding energy, the emission wavelength and the oscillator strength as functions of the structural parameters (the dot height, the barrier thickness between the coupled wurtzite ZnO QDs and Mg content x in the barrier layers) is calculated in detail. The results elucidate that Mg content have a significant influence on the exciton state and optical properties of ZnO coupled QDs. When Mg content x increases, the strong built-in electric field increases and leads to the redshift of the effective band gap of the Mg x Zn 1− x O layer. These theoretical results are useful for design and application of some important photoelectronic devices constructed by using ZnO strained QDs.

Exciton states and optical transitions in InGaN/GaN quantum dot nanowire heterostructures: Strong built-in electric field and dielectric mismatch effects

Considering the strong built-in electric field (BEF), dielectric-constant mismatch and 3D confinement of the electron and hole, the exciton states and interband optical transitions in [0 0 0 1]-oriented Ga-rich wurtzite In x Ga 1− x N/GaN strained quantum dot (QD) nanowire heterostructures are investigated theoretically using a variational approach under the effective mass approximation. 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 radiative decay time. Our calculations show that the radiative decay time of the redshifted transitions is large and increases almost exponentially when the QD height increases, which is in good agreement with the previous experimental and theoretical results.

Optical properties of exciton confinement in wurtzite ZnO/MgxZn1−xO coupled quantum dots

Abstract Based on the framework of effective-mass approximation and variational approach, optical properties of exciton are investigated theoretically in ZnO/Mg x Zn 1− x O vertically coupled quantum dots (QDs), with considering the three-dimensional confinement of electron and hole pair and the strong built-in electric field effects due to the piezoelectricity and spontaneous polarization. The exciton binding energy, the emission wavelength and the oscillator strength as functions of the different structural parameters (the dot height and the barrier thickness between the coupled wurtzite ZnO QDs) are calculated with the built-in electric field in detail. The results elucidate that structural parameters have a significant influence on the exciton state and optical properties of ZnO coupled QDs. These results show the optical and electronic properties of the quantum dot that can be controlled and also tuned through the nanoparticle size variation.

Barrier width dependence of the donor binding energy of hydrogenic impurity in wurtzite InGaN/GaN quantum dot

Within the framework of the effective-mass approximation, the barrier width dependence of the donor binding energy of hydrogenic impurity in a cylindrical wurtzite (WZ) InGaN/GaN strained quantum dot (QD) is calculated by means of a variational procedure, considering the strong built-in electric field effect due to the spontaneous and piezoelectric polarizations. Numerical results show that the built-in electric field and the donor binding energy of the impurity located at any growth direction position are obviously dependent on the barrier width in WZ In0.1Ga0.9N/GaN strained QD with a small barrier width (&amp;lt;8 nm). However, the built-in electric field and the donor binding energy of the impurity located at any growth direction position are insensitive to the barrier width in WZ In0.1Ga0.9N/GaN strained QD with a large barrier width (&amp;gt;8 nm). Moreover, the donor binding energy of the impurity located at the right boundary of the QD is independent of the barrier width with any dot height and indium composition when the barrier width is large (&amp;gt;8 nm).

Influence of strain on hydrogenic impurity states in a GaN/Al x Ga1−x N quantum dot

Within the effective-mass approximation, we calculated the influence of strain on the binding energy of a hydrogenic donor impurity by a variational approach in a cylindrical wurtzite GaN/AlxGa1−xN strained quantum dot, including the strong builtin electric field effect due to the spontaneous and piezoelectric polarization. The results show that the binding energy of impurity decreases when the strain is considered. Then the built-in electric field becomes bigger with the Al content increasing and the binding energy of hydrogenic donor impurity decreases when the Al content is increasing. For dot height L < 2 nm, the change of the binding energy is very small with the Al content variety.

Exciton states and interband optical transitions in ZnO/MgZnO quantum dots

Within the framework of the effective-mass approximation, the exciton states confined in wurtzite ZnO/MgZnO quantum dot (QD) are calculated using a variational procedure, including three-dimensional confinement of carriers in the QD and the strong built-in electric field effect due to the piezoelectricity and spontaneous polarizations. The exciton binding energy and the electron–hole recombination rate as functions of the height (or radius) of the QD are studied. Numerical results show that the strong built-in electric field leads to a remarkable electron–hole spatial separation, and this effect has a significant influence on the exciton states and optical properties of wurtzite ZnO/MgZnO QD.

The donor bound exciton states in wurtzite GaN quantum dot

Based on the effective mass approximation, the donor bound exciton states in a wurtzite (WZ) GaN/AlGaN quantum dot (QD) are investigated by means of a variational method, including the strong built-in electric field effect due to the spontaneous and piezoelectric polarizations. Numerical results show that the donor bound exciton binding energy is highly dependent on the impurity position and QD size. In particular, we find that the donor bound exciton binding energy is insensitive to dot height when the impurity is located at the right boundary of the WZ GaN/AlGaN QD with large dot height.

Exciton states in wurtzite InGaN/GaN quantum wells: Strong built-in electric field and interface optical-phonon effects

Considering the strong built-in electric field (BEF) effects and large exciton–phonon interactions, we investigate the exciton states confined in an InGaN/GaN single quantum well (QW) by using the Lee–Low–Pines variational method. We find that the exciton state modification caused by the exciton–phonon interactions is remarkable. The exciton energy shift due to exciton–phonon interactions increases monotonically if the well width increases. With increasing the In fraction, the exciton energy shift firstly increases to a maximum, then decreases. The BEF has a significant influence on the exciton states in a QW with large well width. The physical reasons have been analyzed in detail. Good agreement for the zero-phonon peak energies and the Huang–Rhys factor has been obtained between our numerical results and the corresponding experimental measurements.

Linear and Nonlinear Intersubband Optical Absorptions andRefractive Index Changes in InGaN Strained Single Quantum Wells: StrongBuilt-inElectric Field Effects

Considering the strong built-in electric field (BEF) effects due tothe spontaneous and piezoelectric polarizations, the intersubbandoptical absorptions and refractive index changes for anInxGa1−xN/AlyGa1−yN strained single quantum well arestudied theoretically within the framework of the density matrixmethod and effective-mass approximation. The linear, third-ordernonlinear and total absorption coefficients and refractive indexchanges are calculated as a function of the incident opticalintensity and photon energy. Our results show that both the incidentoptical intensity and the strong BEF have great influence on thetotal absorptions and refractive index changes. The results aresignificant for designing some important photodetectors and thephotonic crystal devices with adjustable photonic band structures.

Influence of the built-in electric field on luminescent properties in self-formed single-GaN/Al xGa 1−xN quantum dots

Based on the framework of effective-mass approximation and variational approach, the luminescent properties are investigated theoretically in self-formed wurtzite GaN/Al x Ga 1− x N single-quantum dots (QDs). Considering the three-dimensional (3D) confinement of electron and hole pair and the strong built-in electric field effects, the exciton binding energy, the emission wavelength and the oscillator strength are calculated with and without the built-in electric field in detail. The results elucidate that the strong built-in electric field has a significant influence on luminescent properties of GaN/Al x Ga 1− x N QDs.

Parameter-dependent third-order susceptibility of InxGa1−x N/GaN parabolic quantum dots

The electron states confined in wurtzite InxGa1−xN/GaN strained quantum dots (QDs) have been investigated in the effective-mass approximation by solving the Schrodinger equation, in which parabolic confined potential and strong built-in electric field effect (due to the piezoelectricity and spontaneous polarization) have been taken into account. The real part Reχ(3) (0,0,ω) and the imaginary part Im χ(3) (0,0,ω) of the third-order susceptibility deseribe quadratic electro-optic effects and electro-absorption process of the QDs respectively. And both of them have been calculated in directions parallel and vertical to z axis. Furthermore, the study shows Re χ(3) (0,0,ω) and Im χ(3) (0,0,ω) increase under resonant conditions with the QDs’ radius and height increase, and the same results occur when the content increase. In addition, the resonant position shift to the lower energy region when the parabolic frequencies increase.

Second-order nonlinear optical susceptibilities induced by built-in electric field in wurtzite nitride double quantum wells

Based on the density matrix method and the iterative treatment, the second-harmonic generation (SHG) susceptibility of a wurtzite nitride coupling quantum well (CQW) with strong built-in electric fields have been theoretically investigated. The effect of the band non-parabolicity effect has been taken into account. A typical wurtzite GaN/InxGa1–xN CQW are chosen to perform numerical calculations. The localized properties of the electronic ground state and the low-excited states in the system are analyzed in detail. The calculated SHG coefficients reach the order of magnitude of 10−7m/V, which is two-order larger than the corresponding values in wurtzite single quantum wells. Moreover, it is confirmed that the SHG coefficients are not monotonic functions of the well width, barrier width and the doped concentration of the CQW systems, but have complicated dependent relations on them. The reasons resulting in these characteristics can be attributed to the intense competition between the strong built-in electric field effect and the quantum size effect for the electronic confined situation in the wurtzite CQWs. The calculated results also show that a strong SHG effect can be realized in the nitride CQW by choosing a group of optimized structural parameters and doped fraction.

Built-in electric field effect on hydrogenic impurity in wurtzite GaN/AlGaN quantum dot

The binding energy of a hydrogenic donor impurity in a wurtzite (WZ) GaN/AlGaN quantum dot (QD) is investigated, including the strong built-in electric field effect due to the spontaneous and piezoelectric polarizations. Numerical results show that the strong built-in electric field induces an asymmetrical distribution of the donor binding energy with respect to the center of the QD. The donor binding energy is insensitive to dot height when the impurity is located at the right boundary of the QD with large dot height.