Wide-gap Layers Research Articles

Fe3O4@ZnO Core-Shell Nanoparticles—a novel facile fabricated magnetically separable photocatalyst

The current study proposes a simple and inexpensive technique to synthesise core–shell Fe3O4@ZnO as a promising photocatalyst. An easily scalable original procedure was developed based on sequential and simultaneous addition of Zn2+ and OH– ions to Fe3O4 cores, and preheating and subsequent heating of the reaction medium. The structure of Fe3O4@ZnO core–shell nanoparticles was found to be a sequence of layers including magnetite (Fe3O4), maghemite (γ-Fe2O3), goethite (α-FeOOH), and zinc oxide (ZnO) in (0001) orientation. This layer sequence ensures a smooth transition from the Fe3O4 of the core to the ZnO shell. The synthesis conditions affect layer density and thickness, which can be easily used in adjusting Fe3O4@ZnO properties.Photocatalytic degradation of a model persistent dye (naphthol green B) in the presence of Fe3O4@ZnO nanoparticles reached promising 77 % in 60 min under UV radiation, while Fe3O4 cores showed only 20 %. The wide-gap ZnO layer on the surface of the narrow-gap Fe3O4 provides charge separation and suppresses electron-hole recombination, which drastically increases photocatalytic activity of the material. Separation of the photocatalyst with a magnet showed the same result as that with a centrifuge. All of these findings lead to inexpensive fabrication, effective performance, low toxicity and simple separation of Fe3O4@ZnO.

Electrical and Optical Properties of Thin-Film Wide-Gap Metal Oxides Sno 2, In 2 O 3 , Ito , Cdo , Moo 3

The electrical properties of the created doped and undoped wide-gap layers of metal oxides SnO 2, In 2 O 3 , ITO , CdO , MoO 3 were studied. Depending on technological factors. The optimal growth conditions for these semiconductor materials have been determined. The study of the electro physical and optical properties of the resulting oxide films confirmed the stability of the operational parameters of oxide materials SnO 2, In 2 O 3 , ITO , CdO , MoO 3 . The AFM method showed, along with the oxides SnO 2, In 2 O 3 , ITO , CdO , the oxide of pure MoO3 has The highest transmittance is in the visible region of the spectrum , where the absorption edge shifts toward higher wavelengths.

Stochastic simulation of electron transport in a strong electrical field in low-dimensional heterostructures

Abstract In this paper we develop a stochastic simulation algorithm for electron transport in a DA-pHEMT heterostructure. Mathematical formulation of the problem of electron gas transport in the heterostructure in the form of a coupled system of Poisson, Schrödinger and kinetic Boltzmann equations is given. A Monte Carlo model of electron transport in DA-pHEMT heterostructures which accounts for multivalley parabolic band structure, as well as relevant formulas for calculating electron scattering rates and scattering phase functions on polar optical, intervalley phonons and on impurities are developed. The results of a computational experiment involving the solution of the system of Poisson–Schrödinger–Boltzmann equations for the AlGaAs/GaAs/InGaAs/GaAs/AlGaAs heterostructure are presented. The distribution of electrons by energy subband in the main and satellite valleys and the field dependences of the electron drift velocity in each valley are calculated. It was discovered that there is no spatial transfer of electrons into wide-gap AlGaAs layers due to high barriers created by modulated-doped impurities. A comparative analysis of the electron drift velocities in the studied DA-pHEMT heterostructures and in the unstrained layer of the InGaAs is given.

Photovoltaic AlGaAs/GaAs devices for conversion of high-power density laser (800–860 nm) radiation

Photovoltaic converters of high-power (λ = 800–860 nm, ELR = 150–550 W/cm2) laser radiation (PhotoVoltaic Laser Power Converters – PVLPCs) based on AlGaAs/GaAs heterostructures grown by metalorganic vapor-phase epitaxy have been developed. To increase the output voltage, the space charge region of p-GaAs/n-AlхGa1-хAs heterojunction was shifted to the n-AlхGa1-хAs wide-gap layer with a gradual “x”. The technology for embedding a rear reflector based on TiOx/SiO2/Ag into the photovoltaic converter structure by “transferring” the heterostructure to a supporting substrate and the method of bonding to form a monolithic structure of the PVLPC has been elaborated. To operate the heterostructures at an increased power density of laser radiation, they have been designed to eliminate possible potential barriers at the heterointerfaces and the frontal contact topology with a high density of the metal grid (50 μm and 125 μm contact pitch are under consideration) has been employed. PVLPCs with efficiency of 62% were obtained at the power density of monochromatic radiation (λ = 850 nm) ELR = 170 W/cm2. The finger pitch of 50 μm allows keeping efficiency more than 56% even with a fivefold increase in laser radiation (up to ELR = 500 W/cm2).

Influence of thickness and temperature on photoelectric properties of p-CdTe-nCdS and pCdTe-CdSe heterostructures

In this paper, we study the photoelectric properties of pCdTe-nCdS and pCdTe-nCdSe-based film heterostructures. It is shown that the high value of the electron diffusion length in pCdTe slick (pellicle) is due to the presence of a built-in field in it. When studying the effect of temperature on the spectral characteristics, it was found that with increasing temperature, the maximum photosensitivity of the heterostructure shifts towards longer wavelengths of light. The shift in the photosensitivity maximum is explained by changes in the band gap of cadmium telluride. As shown, the accumulation coefficient increases with raising thickness of the wide-gap layer. It has been established that the short-wavelength edge of the photosensitivity of the pCdTe-nCdS structure begins to increase sharply at a photon energy of hv ≤ 2,3 eV, while the photosensitivity of the pCdTe-nCdSe structure already at a photon energy has a significant value ~ 100 μA/mV.

Photoelectric properties of solar cells based on pCdTe-nCdS and pCdTe-nCdSe heterostructures

In this paper, we study the photoelectric properties of pCdTe-nCdS and pCdTe-nCdSe-based film heterostructures. It is shown that the high value of the electron diffusion length in pCdTe slick (pellicle) is due to the presence of a built-in field in it. The shift in the photosensitivity maximum is explained by changes in the band gap of cadmium telluride. As shown, the accumulation coefficient increases with raising thickness of the wide-gap layer. It has been established that the short-wavelength edge of the photosensitivity of the pCdTe-nCdS structure begins to increase sharply at a photon energy of hv ≤ 2,3 eV, while the photosensitivity of the pCdTe-nCdSe structure already at a photon energy has a significant value ∼ 100 μA/mV.

Interlayer excitonic insulator in two-dimensional double-layer semiconductor junctions: An explicitly solvable model

Excitonic insulators conduct neither electrons nor holes but bound electron-hole pairs, excitons. Unfortunately, it is not possible to inject and detect the electron and hole currents independently within a single semiconducting layer. However, interlayer excitonic insulators provide a spatial separation of electrons and holes enabling exciton current measurements. The problem is that the spatial separation weakens electron-hole pairing and may lead to interlayer exciton disassociation. Here we develop an explicitly solvable model to determine an interlayer separation that is strong enough to prevent electron and hole hopping across the layers but still allows for electron-hole pairing sufficient for transition into an interlayer excitonic insulator state. An ideal junction to realize such a state would comprise a pair of identical narrow-gap two-dimensional semiconductors separated by a wide-gap dielectric layer with low dielectric permittivity. The present study quantifies parameters of such a junction by taking into account interlayer coherence effects.

Study of the Crystalline State of MBE (013)HgCdTe/CdTe/ZnTe/GaAs Heterostructure Layers by the Second Harmonic Generation Method

The crystal perfection of HgCdTe layers of heterostructures grown on (013)GaAs substrates with ZnTe and CdTe buffer layers and orientation rotation in the plane(angle φ) and perpendicular to the growth direction (angle theta) were studied by the method of second harmonic generation. A change in the angle φ for CdTe layers was observed depending on the GaAs substrate orientation and its non-monotonic change throughout thickness in the MCT layer of constant composition and graded widegap layers at its boundaries. An increase in the angle theta was observed when growing the upper graded widegap MCT layer. The absolute value of the angle theta can be used for a qualitative assessment of the crystalline perfection of the HgCdTe layers. Keywords: crystals of the sphalerite class, second harmonic, azimuthal angular dependencies, crystal perfection, CdxHg1-xTe heterostructures.

Исследование кристаллического состояния слоев молекулярно-лучевой эпитаксии гетероструктур (013)HgCdTe/CdTe/ZnTe/GaAs методом генерации второй гармоники

The crystal perfection of HgCdTe layers of heterostructures grown on (013)GaAs substrates with ZnTe and CdTe buffer layers and orientation rotation in the plane(angle φ) and perpendicular to the growth direction (angle θ) were studied by the method of second harmonic generation. A change in the angle φ for CdTe layers was observed depending on the orientationof the GaAs substrate and its non-monotonic change throughout thickness in the MCT layer of constant composition and graded widegap layers at its boundaries. An increase in the angle θ was observed when growing the upper graded widegap MCT layer. The absolute value of the angle θ can be used for a qualitative assessment of the crystalline perfection of the HgCdTe layers

Reducing Shockley–Read–Hall recombination losses in the depletion region of a solar cell by using a wide-gap emitter layer

The conversion efficiency of a solar cell depends on the degree of non-radiative recombination, and thus efficiency improvements can also be achieved by reducing Shockley–Read–Hall (SRH) recombination losses. This type of loss depends not only on the crystal quality, but also on the device structure. A clear separation of the contributions of these factors would improve our understanding of the control of non-radiative recombination. In this work, we discuss the reduction of the integrated SRH recombination rate in the depletion region (USRHdep) by changing the emitter material, instead of fabricating a base layer with a higher crystal quality. First, we theoretically show that, by employing a suitable n-InGaP/p-GaAs heterojunction structure instead of a GaAs p–n homojunction, the integrated USRHdep can be reduced, because a significant part of the depletion region in the heterojunction is located in the wide-gap emitter material, which has a lower intrinsic carrier density. Then, the effective SRH recombination coefficient in the depletion region (Adep¯) is obtained from experiments, and the effect of the structural modification on USRHdep is analyzed. We are able to clearly assess the effect of the heterojunction structure on the non-radiative recombination because the grown samples exhibit the same radiative recombination loss. The analysis reveals that, in suitable heterojunction solar cells with an emitter layer containing a low intrinsic carrier concentration, Adep¯ (and thus also USRHdep) is effectively reduced.

Potential high efficiency of GaAs solar cell with heterojunction carrier selective contact layers

Advanced configurations of gallium arsenide (GaAs) solar cells are proposed, using hydrogenated amorphous silicon (a-Si: H) and zinc oxide (ZnO) layers as passivation and carrier selective contacts (CSC), as alternatives to the conventional epitaxial GaInP CSC layers. The cell operation is simulated based on the AFORS-HET simulation program. The results show that with a wide-gap ZnO window layer, single-junction GaAs solar cells can reach a higher short-circuit current density (Jsc) and open-circuit voltage (Voc) than that obtained with conventional GaInP CSC layers without affecting the fill factor (FF). Notably, wide-gap and n-type doping ZnO layers show great potential as electron selective contacts in GaAs solar cells with high barriers at the valence band for blocking hole-transport. An efficiency of 30% is reached for a single-junction GaAs cell with a front ZnO electron selective contact. A 4-terminal GaAs/c-Si tandem solar cell configuration with an estimated conversion performance of 35.16% is proposed. These results indicate enormous potential for the development of low-cost and high-efficiency GaAs-based solar cells in the future.

Advanced architecture ofCdHgTe photodiode for detection of weak infrared radiation

The dark currents of CdHgTe photodiodes based on the advanced architecture, which isused inthreshold FPAfordetection ofweak infrared radiation,have been analyzed. Theopposite conductivity regions areformedof in wide-gap layers, thatreducesthe contribution of theSRHgeneration-recombination currents of to the total dark current.By using various compositions layers with transition sublayers reducing surface recombination at the interfaces, one canre-duce the surface mechanismcontributionto the total dark current of the photodiode. Finally, due to the correct choice of the absorption layer composition and wide-gap layer parameters, dark current in the space charge region ismuch lower than аdiffusion currentcausing by Auger mechanismin the absorption region for a given cutoff wavelength.

Влияние имплантации ионов As-=SUP=-+-=/SUP=- и последующего отжига на электрические свойства приповерхностных слоев варизонных пленок n-Hg-=SUB=-0.78-=/SUB=-Cd-=SUB=-0.22-=/SUB=-Te

Films of n-Hg0.775Cd0.225Te with near-surface wide-gap layers were grown by molecular beam epitaxy on Si (013) substrates. To measure the admittance, metal – insulator – semiconductor (MIS) structures were fabricated on the basis of the as-grown HgCdTe film, films after implantation, and films after implantation and annealing. Using techniques that take into account the presence of graded-gap layers and slow interface states, the main parameters of the near-surface layers of HgCdTe films have been determined after the technological procedures used to create photodiodes.

Temperature limitations for stimulated emission in 3–4 μ m range due to threshold and non-threshold Auger recombination in HgTe/CdHgTe quantum wells

We report on the stimulated emission (SE) from HgTe/CdHgTe quantum well (QW) heterostructures up to 240 K at 3.7 μm wavelength. Based on the temperature dependence of the SE threshold, a total Auger recombination (AR) coefficient of 10−27 cm6/s has been deduced for HgTe/CdHgTe QWs, which is much lower than that for bulk HgCdTe with the same bandgap and indicates suppression of (threshold) AR processes due to the symmetry of carrier dispersion curves. We demonstrate that QW-specific, non-threshold AR contributes strongly to the temperature quenching of laser action from HgTe/CdHgTe QWs. We expect, however, that the above processes may be partially suppressed via introduction of wide-gap CdHgTe barrier layers with a [Cd] fraction of 80% or higher. In this case, lasing up to at least 270 K at 3.7 μm wavelength seems feasible.

Wide-gap ZnO layer as electron-selective front contact for single-junction GaAs solar cells

Herein, a single-junction GaAs solar cell using a wide-gap n-type doped ZnO layer as a front electron-selective contact is proposed. An atomic layer deposition approach was used to deposit a thin ZnO layer on a GaAs substrate, which demonstrated excellent surface passivation properties by significantly reducing the native oxide statuses of the Ga–O and As–O bonds at the GaAs surface. Additionally, n-type doped ZnO layer acted as a front electron-selective contact layer that blocked hole transport and promoted electron collection at the front side. A wide-gap ZnO window layer was shown to enhance cell absorption in the short wavelength range and increase built-in potential, and was proven to be a suitable alternative to the conventional AlGaAs front window layers used in highly efficient single-junction GaAs cells.

NBn-Photodiode Based on InAsSb/AlAsSb Alloys with a Long-Wavelength Cutoff of 5 μm

The results of studying the photoelectric characteristics of an nBn structure are presented. The photodiodes based on such structures may have a cutoff wavelength as high as 5 μm. Calculations based on these results show that such photodiodes have a threshold photosensitivity comparable with that of traditional analogs. An energy band diagram of the nBn structure is proposed based on experimental results and theoretical estimations; this diagram makes it possible to estimate the effect of potential barriers in the valence band of the wide-gap layer and at its boundaries with narrow-gap layers on the nBn-based photodiode sensitivity. The experimental results of studying the dependence of the photocurrent thermal-activation energy on the photodiode bias turned out to be important for developing the model.

Analysis of characteristics of photodetectors based on InGaAs heteroepitaxial structures for 3D imaging

The 320 × 256 focal plane arrays based on р + -B–n-N + tetralayer heterostructures with a wide-gap barrier layer have been investigated. The heterostructures with a narrow-gap n-InGaAs absorbing layer were grown by means of metalorganic vapor phase epitaxy on InP substrates. The band discontinuity between the In0.53Ga0.47As absorbing layer and the In0.52Al0.48As barrier layer is removed by growing a thin four-component n-AlInGaAs layer with the bandgap gradient variation. Delta-doped layers included into the heterostructures make it possible to lower the barrier in the valence band and eliminate the nonmonotonicity of energy levels. The experimental study of the dark current has been performed. It has been revealed that the average value of the dark current does not exceed 10 fA for the photodiode arrays with a pitch of 30 μm.

Giant magnetoresistance of strong magnet–polymer heterostructures with a wide band gap

We present the results of studying the effect of a weak external magnetic field on the conductivity of heterostructures containing a wide-gap polymer layer. A set of experiments is singled out for switching the metal–dielectric conductivity, controlled by a weak magnetic field. Evidence of the reliability and repeatability of key experiments are reviewed, and the effect of possible artifacts on switching of the conductivity in the heterostructures under study is discussed.

Low concentrator hetero-junction microcrystalline silicon solar cells

Multi-junction silicon-based thin-film solar cells are attractive materials for further cost-reduction and high efficiency. Meanwhile, it is also well considered that a concentrator solar cell is another alternative approach to enhance the conversion efficiency. In concentrator solar cells, the photocurrent linearly increases with the concentration ratio of incident light. At the same time, the open-circuit voltage (Voc) of solar cells increases logarithmically with the photocurrent. This leads to an increase in efficiency with increasing sunlight intensity.We proposed a novel hetero-junction structure microcrystalline silicon (μc-Si:H) solar cell structure using wide-gap microcrystalline silicon oxide (μc-Si1−xOx:H) as p-layer and it has some advantages over conventional homo-junction μc-Si:H solar cells under low concentrations. It was observed that wide-gap doped layers can reduce carrier recombination rate especially in p-layer and at the p/i interface and Voc enhancement with increasing light intensity improves as the band gap of p-layer is increased. Our best solar cell has efficiencies of 9.2% at 1sun and 10.4% at 11.8suns. We also investigated the degradation behavior of hetero-junction μc-Si:H solar cells. The degradation in efficiency for this type of solar cell was less than 6%. Therefore, hetero-junction μc-Si:H solar cell is the promising alternative for low-light concentration.

Electrical and electroluminescent properties of InAsSb-Based LEDs (λ = 3.85–3.95 μm) in the temperature interval 20–200°C

The temperature dependences of the electrical and electroluminescent properties of InAsSbP/InAsSb/InAsSbP heterostructure LEDs (λ ≈ 3.8−4.0 μm) are studied in the temperature interval 20–200°C. It is shown that the radiation power decreases with increasing temperature in a superexponential manner and that this decrease is associated primarily with a rise in the rate of Auger recombination. The position of the maximum in the radiation spectrum varies with temperature nonmonotonically, since radiative recombination is observed both in the active region and in the wide-gap layer. At room temperature, current through the heterostructure is tunneling current irrespective of the applied voltage polarity. As the temperature rises, either the thermal emission of charge carriers appears (direct bias) or the diffusion current becomes significant (reverse bias).