Computational modelling of the refractive index and reflectivity of a quantum well solar cell

In this paper the modelling of the refractive index and reflectivity of a quantum well solar cell (QWSC) is theoretically developed and computationally analysed. The model is based on the Modified Single Effective Oscillator model combined with the Fresnel equation. The model takes into consideration the effects of the design parameters including concentration levels, structural properties of the device (well length, etc.), operating temperature and electric field effects (due to doping concentration). Prior to analysis the authors show the model to have a solid foundation given it generates accurate results which match with experimental data for an AlGaAs/GaAs triple heterojunction cell. This justifies and promotes further work where the theoretical and computational analysis is repeated for a QWSC structure. In the case of the QWSC the results generated are for a bare AlGaAs/GaAs cell and the same cell with a ZnS anti-reflection coating. The results generated show promise for the new model however experimental verification is required. The analysis is performed for AM 1.5G spectrum. The model is intended to be an aid to QWSC designers.     

Intermixing of InP-based multiple quantum wells for integrated optoelectronic devices

The intermixing characteristics of three widely used combinations of InP-based quantum wells (QW) are investigated using the impurity-free vacancy disordering (IFVD) technique. We demonstrate that the bandgap energy shift is highly dependent on the concentration gradient of the as-grown wells and barriers, as well as the thickness of the well, with thinner wells more susceptible to interdiffusion at the interface between the barrier and well. According to our results, the InGaAsP/InGaAsP and InGaAs/InP are well suited for applications requiring a wide range of bandgap values within the same wafer. In the case of the InGaAs/InGaAsP system, its use is limited due to the significant broadening of the photoluminescence spectrum that was observed. The effect of the top InGaAs layer over the InP cladding is also investigated, which leads to a simple way to obtain three different bandgaps in a single intermixing step.     

无规则非均匀折射率场描述

在大气或流动介质中的折射率分布是无规则非均匀的,由于流动和温度变化的影响,无法用通常的梯度折射率公式来描述。提出采用自适应网格方法来描述无规则非均匀折射率场,将数据以动态八叉树结构存放在内存中,给出了网格划分的折射率判据和折射率梯度判据,并采用插值法来计算光线传播路径上的折射率和折射率梯度。以一种可以用公式描述的梯度折射率棒为例,在折射率阈值为0.005、折射率梯度阈值为0.000 5的条件下,建立了自适应网格并求出了一些位置处的折射率及其梯度,计算结果的均方根误差小于7×10－5,可以用于光线追迹。     

A revised ideal model for AlGaAs/GaAs quantum well solar cells

Quantum well solar cells (QWSCs) are heterostructure devices intended to achieve higher efficiencies than conventional cells. This paper extends a previous model for QWSC current-voltage characteristics by revising the equations for the absorbed flux and by introducing expressions to calculate radiative recombination coefficients and well effective densities-of-states. This revised model is in agreement with previous experimental results for AlGaAs/GaAs. Since the revised model incorporates detailed balance calculations, its predictions are consistent with the efficiency restrictions of this theory. The revised model, however, does predict efficiency enhancements for QWSCs in some configurations if non-radiative recombination is dominant, even in such a poor QWSC material as AlGaAs/GaAs.     

Photoresponse study of normal incidence detection in p-type GaAs/AIGaAs multiple quantum wells

We studied p-type GaAs/AIGaAs multiple quantum well (MQW) materials as a possible alternative to the current n-type GaAs/AIGaAs MQWs for infrared detection. The advantage of p-type MQWs is that absorption of infrared radiation at normal incidence is not selection rule forbidden as it is for the n-type. We have verified that significant photoresponse occurs at normal incidence in p-type MQWs. We studied changes in the photoresponse spectrum as a function of well width and temperature. The MQW heterostructures were designed to use bound to continuum intersubband absorption in the GaAs valence band and to have a peak photoresponse near 8 μm. The photoresponse spectrum was compared to the first theoretical model of the bound to continuum absorption in p-type GaAs/ AlGaAs MQWs. The theoretical absorption curve was found to be in good qualitative agreement with the experimental results.     

Well-width dependence of the phase coherent photorefractive effect in ZnSe quantum wells

We report on the well-width dependence of the phase coherent photorefractive (PCP) effect in ZnSe/ZnMgSe single quantum wells (QWs) using 90 fs light pulses. The experiments are performed in a four-wave-mixing configuration at temperatures between 25 and 65 K. The ZnMgSe/ZnSe QWs with 10, 5 and 3 nm well width were grown on GaAs substrate using molecular beam epitaxy. With decreasing QW thickness we observe a reduction of the PCP diffraction efficiency. This is attributed to the higher electron energy in small QWs due to the quantum size effect, which leads to a reduced equilibrium density of captured electrons in the QW. With increasing temperature the PCP signal further decreases which is attributed to thermal activation of the QW electrons back to the GaAs substrate.     

On the physics of semiconductor quantum dots for applications in lasers and quantum optics

The progression of carrier confinement from quantum wells to quantum dots has received considerable interests because of the potential to improve the semiconductor laser performance at the underlying physics level and to explore quantum optical phenomena in semiconductors. Associated with the transition from quantum wells to quantum dots is a switch from a solid-state-like quasi-continuous density of states to an atom-like system with discrete states. As discussed in this paper, the transition changes the role of the carrier interaction processes that directly influence optical properties. Our goals in this review are two-fold. One is to identify and describe the physics that allows new applications and determines intrinsic limitations for applications in light emitters. We will analyze the use of quantum dots in conventional laser devices and in microcavity emitters, where cavity quantum electrodynamics can alter spontaneous emission and generate nonclassical light for applications in quantum information technologies. A second goal is to promote a new connection between physics and technology. This paper demonstrates how a first-principles theory may be applied to guide important technological decisions by predicting the performances of various active materials under a broad set of experimental conditions.     

Band-filling effect on the light emission spectra of InGaN/GaN quantum wells with highly doped barriers

We investigate spectra of InGaN/GaN quantum well (QW) light-emitting diode (LED) structures with heavily doped barriers at different excitation levels. We model the spectral shape and energy position in frames of dominating mechanism of free electron recombination accounting for the influence on the potential width of the QW of the random impurity potential penetrating from the doped barriers. The blue shift at high excitation is supposed to be due to the filling of the conduction band with degenerate 2D non-equilibrium electrons. A structure in the emission bands is observed and it is assumed to be a result from step-like 2D density of states in the QW. A good accordance is obtained between the calculated and experimental spectra assuming that the barriers are graded.     

Effects of boron incorporation on the strain and photoluminescence properties of GaAsSb/GaAs quantum wells

Boron-containing GaAsSb/GaAs quantum wells (QWs) with different antimony (Sb) mole fractions were grown by low-pressure metal–organic chemical vapor deposition for the first time. The effects of boron incorporation on the performance of GaAsSb/GaAs QWs are discussed. For samples with low compressive strain, injection of triethylboron can enhance the Sb content and increase the compressive strain, although boron incorporation can lead to a reduction in strain. This effect was less for strained GaAsSb/GaAs QWs, so the compressive strain of these QWs did not vary. Room-temperature photoluminescence emission at 1116 nm with a full-width at half-maximum (FWHM) value of 56 meV was obtained for strained BGaAsSb/GaAs QWs.     

一种新型太阳能电池减反射膜的制备

提高太阳电池的光电转换效率的方法有很多,该文提出了一种折射率不能变化的减反射膜,具有更好的减反射效果,增加了光生载流子的生成数量,从而提高晶硅太阳电池的转换效率。通过硅烷和氨气流量的连续变化,从而使减反射膜的折射率由上至下等速率地增大,可提高电池转换效率0.13%。     

改进长周期波导光栅折射率传感特性的研究

提出了一种附加高折射率覆盖层的长周期波导光栅折射率传感器结构.通过模拟外折射率变化所引起的高阶模式等效折射率的改变以及该高阶模式与基模相耦合的谐振波长的漂移,研究了长周期波导光栅对外界环境折射率的传感特性.模拟表明,高折射率外覆盖层的加入会使得原有的覆盖层高阶模式发生重组,高阶模式等效折射率和模式耦合的谐振波长随之发生跳变.此时,长周期波导光栅外折射率传感器的敏感度和工作范围将极大地提高.     

Structural and optical properties of AlxGa1-xN/AlyGa1-yN multiple quantum wells for deep ultraviolet emission

The AlxGa1-xN/AlyGa1-yN multiple quantum well (MQW) structure for deep ultraviolet emission has been grown on sapphire by metal organic chemical vapor deposition (MOCVD).High resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM), and cath-odoluminescence (CL) are used to characterize the structural and optical properties of MQWs, respectively.Clear step flows can be observed in the AFM image indicating a two-dimensional growth model.There are many cracks on the surface of the MQW structure because of the high tensile stress.HRXRD shows multiple satellite peaks to the 2rid order.The HRXRD simulation shows that the MQW period is about 11.5 nm.The emission peak of AlxGa1-xN/AlyGa1-yN MQWs is about 295 nm in the deep ultraviolet region from the CL spectra.     

Theoretical study of InAs/GaAs quantum dots grown on [11k] substrates in the presence of a magnetic field

Eight-band k.p theory including strain and piezoelectric effects are employed to calculate the strain distribution and electron and hole energy levels of InAs/GaAs quantum dots grown on [11k] substrates in the presence of an external magnetic field. Height of the dot determines how the increasing of k influences isotropic part of the strain tensor while biaxial part of the strain tensor is always reduced with increasing k. Because of the reduced symmetry of high index surfaces, influence of piezoelectric effect on the electronic structure becomes more dominant with increasing k. Electron energy levels are influenced by the isotropic part of the strain compared to the hole energy levels, where strong heavy hole–light-hole mixing is observed. For the dots grown on the [11k] surfaces magnetic field has smaller influence on the electron and hole energy levels as compared to the referent case.     

Investigation of scalability of In0.7Ga0.3As quantum well field effect transistor (QWFET) architecture for logic applications

In this paper, the scalability of In_0.7Ga_0.3As QWFET is investigated using two-dimensional numerical drift-diffusion simulation. Numerical drift-diffusion simulations were calibrated using experimental results on short-channel In_0.7Ga_0.3As QWFETs [7] to include the effects of velocity overshoot. Logic figures of merit (sub-threshold slope, saturated threshold voltage, drain induced barrier lowering, I_ON/I_OFF ratio over a specified gate swing, effective injection velocity and intrinsic switching delay) extracted from the numerical simulations are in excellent agreement with the experimental data. Three alternate QWFET device architectures are proposed and thoroughly investigated for 15 nm node and beyond logic applications. Amongst them, double-gate In_0.7Ga_0.3As QWFET shows the best scalability in terms of logic figures of merit, thus making it an ideal candidate for the design and demonstration of the ultimate scaled transistor.     

基于Matlab的太阳能电池数学模型及输出特性分析

该文主要讨论了太阳能电池的数学模型、温度及光照变化时4个典型参数Voc、Isc、Vm、Im的计算方法及仿真结果。以无锡尚德3909电池在25°C 1000W和4209电池在30°C、的测试数据为参考数据,分别给出了温度为25°C时800W、200W和光照在1 000W/m2时40和60°C两种情况下基于数学模型的Voc、Isc、Vm、Im的计算结果和基于Matlab的仿真图形,分析了输出特性曲线随温度和光照变化的变化规律,结果表明计算及仿真结果和实际测试数据基本相符。     

基于数字全息层析术对单模光纤折射率三维分布的重构研究

提出了一种基于数字全息层析术的数字重构方法 。针对单模光纤(SMF)的折射率分布具 有轴对称性的特点,可仅根据在任一与光纤轴向垂直的旋转角度下从数字全息图再现出的相 位分布, 采用层析算法重构出与SMF轴向垂直的折射率断层分布。通过获取SMF的折 射率断层分 布,就可获取其折射率三维分布。与以往测量SMF折射率分布的方法相比,本文方法 具有对被测样 品无损、测量方法简单及测量速度快等特点。理论分析与光学实验结果均验证了本文所提方 法的有效性。     

Influence of oligothiophene-functionalized co-sensitizer on the electron injection efficiency for multiple dye-TiO2 interface

Co-sensitizer has been employed in dye-sensitized solar cells (DSSCs) to enhance light harvesting at organic/inorganic heterogeneous. Here, the multiple dyes@TiO₂ interface has been investigated by density functional theory simulations, to explore the role of varied oligothiophene-functionalized co-sensitizers on the electron injection efficiency. In presence of co-sensitizers, the simulated absorption spectra broaden with the increasing of the number of thiophene from 0, 1, to 2. Meanwhile, the co-sensitizer modifies the energy alignment of interface, and influences the electronic coupling between dye and TiO₂. Critically, the ratio of electron-hole recombination and electron injection rates k_rec/k_inj based on Marcus theory for both dye and co-sensitizer decrease significantly with increasing of the number of oligothiophene, resulting in the improved electron injection efficiency. Our result implies that the electron injection efficiency depends on the number of thiophene in co-sensitizer largely, and appropriate number plays an active role in tuning the electronic properties of hybrid heterostructure.     

Determination of the minimum island size for full exciton localization due to thickness fluctuations in Zn1−xCdxSe quantum wells

Fluctuations of the thickness of quantum wells (QWs) of few monolayers are one of the causes of exciton localization. Here, we present the results of the determination of the minimum lateral dimensions of islands produced by thickness fluctuations in Zn_1−xCd_xSe QWs, which cause full exciton localization. We have calculated the localization energy of excitons in the frame of the factorized-envelope approximation. We found that the excitons are well localized in the islands of the QW when their lateral dimensions are larger than ∼15 times the exciton Bohr radius.     

Effect of Ag-doped TiO2 thin film passive layers on the performance of photo-anodes for dye-sensitized solar cells

Ag-doped nanocrystalline structures of TiO₂ in the form of colloidal particles are synthesized by using AgNO₃ solution with Acetyl acetone and diacetyl monoxime modified titanium isopropoxide precursor. Thin layers of undoped and Ag-doped TiO₂ sols are deposited on conducting glass substrate by dip coating technique. The thin films are developed by repeated coating of the sols for 5, 10 and 15 times which results the formation of thin layers with different thickness. These layers are annealed at 450 °C so as to use them as bottom/ seed layers of fabricating photoanodes for dye sensitized solar cells. The optical properties of undoped and Ag-doped TiO₂ thin layers are studied in details. Effective mass model is used to find the particle size and peak wavelength relationship for the nanocrystals of TiO₂. Quantum efficiency of dye sensitized solar cells assembled using the fabricated photoanodes with Ag-doped TiO₂ shows increase in efficiency by about 4% in comparison to the undoped TiO₂ layers. The improved performance is attributed to ease of transportation of electron through passivating layer to the external circuit.     

A digital signal-processing analysis technique for the infrared reflectivity characterization of ion implanted silicon

A new method for the characterization of ion-implanted silicon is proposed. It is based on analysis of the Fourier spectrum of bilinearly transformed infrared reflectance versus wavenumber data of ion-implanted samples. This non-destructive technique has been applied to previously published infrared reflectance data of 〈111〉 and 〈100〉 oriented Si samples which had been implanted with 2.7 MeV phosphorus and 380 keV silicon ions, respectively, and annealed at 500° C for various lengths of time. The refractive index and thickness of the amorphous layer of the as-implanted samples can be measured directly by means of this technique. The position of boundaries between the amorphous, recrystallized and substrate zones, as well as the position of the carrier concentration peak can be determined for the various annealing times. Depending on the annealing time, the recrystallized layer in 〈111〉 silicon has a refractive index which is between 2% and 4% higher than the substrate refractive index, while the difference in refractive index between the amorphous and recrystallized layers is in the order of 5%. In contrast to these results, the presence of the substrate/recrystallized material interface could not be detected in partially recrystallized 〈100〉 silicon by this method, implying that the refractive index step at the substrate/recrystallized material interface is less than 1%. The step in refractive index at the crystalline/amorphous interface in 〈100〉 silicon implanted with a dose of 0.5 x 10¹⁶ cm⁻² silicon ions, was measured to be 12%, and it is reduced to 8% after partial regrowth has occurred. These results confirm the data obtained by a model-based least-squares analysis.