Quantitative measurements of intervalley and carrier-carrier scattering in GaAs with hot (e,A) luminescence

We have used photoluminescence from nonequilibrium electrons recombining at neutral acceptors to quantitatively measure hot electron kinetics in GaAs. Values have been obtained for intervalley scattering rates as a function of electron kinetic energy and the scattering rate of a single nonequilibrium electron in the presence of a sea of thermalized carriers. These measurements demonstrate the power of this new probe of nonequilibrium carrier relaxation in direct gap semiconductors.     

Determination of the LO-phonon and Γ→L intervalley scattering time in GaAs from hot electron luminescence spectroscopy

Both the LO-phonon scattering time and the Γ→L intervalley scattering time for electrons in the conduction band of GaAs are of fundamental importance, and they are needed for the modelling of devices. We measure the steady state distribution of hot electrons in lightly p-doped bulk GaAs under carrier densities of 10¹³–10¹⁴cm⁻³, which is orders of magnitude lower than in pulsed laser experiments. Using a 16×16 k.p Hamiltonian and taking into account the transition matrix elements in a dipole model, we determine the hot electron lifetime from comparison with the experimentally found lifetime broadening. For electrons with a kinetic energy of 100meV to 300meV we obtain τ_LO=(132±10)fs. We also determine the Γ→L intervalley separation as E_ΓL=(300±10)meV. We find Γ→L scattering times around 150fs to 200fs, corresponding to a value for the associated deformation potential of D_ΓL=(9.4±1.5)×10⁸eV/cm.     

Polar optical phonon scattering of charge carriers in alloy semiconductors: Effects of phonon localization

The effects of spatial localization of phonons or their correlation functions to finite distances in alloy semiconductors on polar optical phonon scattering of hot carriers are modeled analytically. Despite the possibility of increased numbers of carrier-scattering-active phonon modes, it is demonstrated that phonon localization in alloys should have little if any effect on the total polar optical scattering rate for charge carriers coupled to equilibrium phonon populations. Further, it is demonstrated that phonon localization may have a beneficial effect on hot carrier transport by reducing the possibility of exciting nonequilibrium phonon populations. These results are obtained without assuming any specific functional form or degree of phonon localization; rather, calculations rely on the inherent orthogonality and mathematical completeness of the classical vibrational modes over the crystal lattice degrees of freedom.     

Light absorption and many‐particle electron–phonon interactions in intermediate band quantum well solar cells

The carrier dynamics under solar excitation in the third generation AlAs‐Ga_0.27In_0.73As quantum well solar cell on an InP substrate is theoretically investigated. The quantum well shows a significantly higher carrier occupation compared with that of a pure bulk sample. Furthermore, the influence of carrier scattering via the interaction with a bath of longitudinal optical phonons stemming from the substrate is taken into account, showing strong intraband and interband relaxation of the carrier distribution toward the lowest subband states. Copyright © 2014 John Wiley & Sons, Ltd.     

The intervalley X6→Γ6, L6 scattering time in GaAs measured by ultrafast pump-probe infrared absorption spectroscopy

A direct measurement of the dynamics of electrons in the X₆ valley for a GaAs crystal by time-resolved absorption spectroscopy is reported for the first time. IR picosecond probe pulses were used to monitor the growth and decay of the population in the X₆ valley subsequent to excitation by a 527 nm pump pulse. The intervalley X₆→Γ₆, L₆ scattering time t_x of 0.70 ± 0.50 ps is determined and the crossection for the X₆→X₇ transition is estimated to be 1.8 × 10⁻¹⁶ cm².     

晶格失配对GaAsSb/InP异质结中合金拉曼散射的温度依赖特性的影响

通过在3～300 K的低温变温拉曼光谱的测量,对不同Sb组分的GaAsSb/InP异质结中的由Sb组分引起的声子非谐效应进行了拉曼散射的研究。实验发现,随着温度的降低,长光学声子峰位向高波数移动,当温度低于100 K时,变化趋于平缓。分别利用三声子模型和四声子模型计算模拟了光学声子和温度的依赖关系,并和实验结果对比发现,与三声子模型相比,四声子模型与实验数据符合更好,表明温度依赖的拉曼散射峰位的变化必须考虑四次声子非谐振动。相对于晶格失配的样品S1（Sb=37.9%）和S3（Sb=56.2%）,获得的声子非谐度在晶格匹配的样品S2（Sb=47.7%）中是最小的,同时通过对声子线宽的研究表明S2中声子寿命是最长的。结合低温光致发光的实验结果,证实了GaAsSb合金晶格振动的声子非谐效应和声子寿命不仅受合金无序散射的影响,同时受到和衬底晶格失配引入的线缺陷和缺陷的声子散射影响。     

Impact of Crystal Surface on Photoexcited States in Organic–Inorganic Perovskites

Despite their outstanding photovoltaic performance, organic–inorganic perovskite solar cells still face severe stability issues and limitations in their device dimension. Further development of perovskite solar cells therefore requires a deeper understanding of loss mechanisms, in particular, concerning the origin and impact of trap states. Here, different surface properties of submicrometer sized CH₃NH₃PbI₃ particles are studied as a model system by photoluminescence spectroscopy to investigate the impact of the perovskite crystal surface on photoexcited states. Comparison of single crystals with either isolating or electron‐rich surface passivation indicates the presence of positively charged surface trap states that can be passivated in case of the latter. These surface trap states cause enhanced nonradiative recombination at room temperature, which is a loss mechanism for solar cell performance. In the orthorhombic phase, the origin of multiple emission peaks is identified as the recombination of free and bound excitons, whose population ratio critically depends on trap state properties. The dynamics of exciton trapping at 50 K are observed on a time‐scale of tens of picoseconds by a simultaneous population decrease and increase of free and bound excitons, respectively. These results emphasize the potential of surface passivation to further improve the performance of perovskite solar cells.     

散射效应对FSO-OFDM系统的影响研究与仿真

为了研究了大气散射效应对光信号的影响,以雨粒子为例,从其物理特性出发,应用Mie散射理论和Joss雨滴谱分布,计算了雨粒子对光波传输的衰减,在此基础上分析了光散射效应对自由空间光通信-正交频分复用系统性能的影响。结果表明,在散射信道中,自由空间光通信-正交频分复用系统的性能主要取决于散射造成的光强度起伏方差,其次为散射造成的光强度衰减系数。     

STUDY ON THE RELATION BETWEEN STRUCTURE AND HOT CARRIER EFFECT IMMUNITY FOR DEEP SUB-MICRON GROOVED GATE NMOSFET’s

Grooved gate structure Metal-Oxide-Semiconductor(MOS) device is considered as the most promising candidate used in deep and super-deep sub-micron region,for it can suppress hot carrier effect and short channel effect deeply.Based on the hydrodynamic energy transoprt model,using two-dimensional device simulator Medici,the relation between structure parameters and hot carrier effect immunity for deep-sub-micron N-channel Mosfet‘s is studied and compared with that of counterpart conventional planar device in this paper.The examined structure parameters include negative junction depth,conventinal planar device in this paper.The examined structure parameters include negative junction depth,concave corner and effective channel length.Simulation results show that grooved gate device can suppress hot carrier effect is strongly influenced by the concave corner and channel length for grooved gate device.With the increase of concave corner,the hot carrier effect in groovd gate MOSFET decreases sharply,and with the reducing of effective channel length,the hot carrier effect becomes large.     

Reversible Photoinduced Phase Segregation and Origin of Long Carrier Lifetime in Mixed‐Halide Perovskite Films

Mixed‐halide hybrid perovskite semiconductors have attracted tremendous attention as a promising candidate for efficient photovoltaic and light‐emitting devices. However, these perovskite materials may undergo phase segregation under light illumination, thus affecting their optoelectronic properties. Here, photoexcitation induced phase segregation in triple‐cation mixed‐halide perovskite films that yields to red‐shift in the photoluminescence response is reported. It is demonstrated that photoexcitation induced halide migration leads to the formation of smaller bandgap iodide‐rich and larger bandgap bromide‐rich domains in the perovskite film, where the phase segregation rate is found to follow the excitation power‐density as a power law. Results confirm that charge carrier lifetime increases due to the trapping of photoexcited carriers in the segregated smaller bandgap iodide‐rich domains. Interestingly, these photoinduced changes are fully reversible and thermally activated when the excitation power is turned off. A significant difference in activation energies for halide ion migration is observed during phase segregation and recovery process. Additionally, the emission linewidth broadening is investigated as a function of temperature which is governed by the exciton–optical phonon coupling. The mechanism of photoinduced phase segregation is interpreted based on exciton–phonon coupling strength in both mixed and demixed (segregated) states of perovskite films.     

Effect of Doping and Excitation Wavelength on Charge Carrier Dynamics in Hematite by Time‐Resolved Microwave and Terahertz Photoconductivity

The charge carrier dynamics of epitaxial hematite films is studied by time‐resolved microwave (TRMC) and time‐resolved terahertz conductivity (TRTC). After excitation with above bandgap illumination, the TRTC signal decays within 3 ps, consistent with previous reports of charge carrier localization times in hematite. The TRMC measurements probe charge carrier dynamics at longer timescales, exhibiting biexponential decay with characteristic time constants of ≈20–50 ns and 1–2 μs. From the change in photoconductance, the effective carrier mobility is extracted, defined as the product of the charge carrier mobility and photogeneration yield, of differently doped (undoped, Ti, Sn, Zn) hematite films for excitation wavelengths of 355 and 532 nm. It is shown that, unlike in conventional semiconductors, donor doping of hematite dramatically increases the effective mobility of the photogenerated carriers. Furthermore, it is shown that all hematite films possess higher effective mobility for 355 nm excitation than for 532 nm excitation, although the time dependence of the photoconductance decay, or charge carrier lifetime, remains the same. These results provide an explanation for the wavelength dependent photoelectrochemical behavior of hematite photoelectrodes and suggest that an increase in photogeneration yield or charge carrier mobility is responsible for the improved performance at higher excitation energies.     

GaAs MESFET旁栅效应与旁栅距的关系研究

旁栅效应是影响 Ga As器件及电路性能的有害寄生效应。本文研究了旁栅阈值电压 Vth SG与旁栅距 LSG的关系 ,发现 Vth SG的大小与 LSG成正比关系 ,并理论探讨了产生这一现象的机制 ,从而验证了旁栅阈值电压与旁栅距关系的有关理论。     

JLNT-FET和IMNT-FET中传导机制对电热特性影响研究北大核心

无结纳米管场效应晶体管(JLNT-FET)和反转模式纳米管场效应晶体管(IMNT-FET)因具有较好的驱动能力和对短沟道效应(SCE)卓越的抑制能力而被关注，自热效应(SHE)作为影响其电热性能的关键问题而被广泛研究。文章基于TCAD数值仿真，通过对环境温度(TA)、接触热阻(Rtc)以及侧墙长度(LS)对体传导的JLNT-FET和表面传导的IMNT-FET的最大晶格温度(TLmax)、最大载流子温度(TCmax)、漏极电流(IDS)和栅极泄漏电流(IG)等器件参数影响的分析，对比研究了JLNT-FET和IMNT-FET中传导机制对电热特性的影响。结果表明，较高的TA、较大的Rtc及较小的LS,都会加剧器件的声子散射，导致严重的SHE。同时，由于传导机制的差异，体传导受界面散射和声子散射影响较小，JLNT-FET具有更好的电热特性。     

深亚微米槽栅PMOSFET结构参数对其抗热载流子效应和短沟道抑制作用的影响

基于流体动力学能量输运模型 ,利用二维仿真软件 Medici对深亚微米槽栅 PMOS器件的结构参数 ,如凹槽拐角、负结深、沟道和衬底掺杂浓度对器件抗热载流子特性和短沟道效应抑制作用的影响进行了研究 .并从器件内部物理机理上对研究结果进行了解释 .研究发现 ,随着凹槽拐角、负结深的增大和沟道杂质浓度的提高 ,器件的抗热载流子能力增强 ,阈值电压升高 ,对短沟道效应的抑制作用增强 .而随着衬底掺杂浓度的提高 ,虽然器件的短沟道抑制能力增强 ,但抗热载流子性能降低     

Organic Transistor Based on Cyclopentadithiophene‐Benzothiadiazole Donor–Acceptor Copolymer for the Detection and Discrimination between Multiple Structural Isomers

Distinguishing structural isomers is a critical and challenging task for biotechnology, chemical industry, and environmental monitoring. Approaches currently available are limited in terms of selectivity and simplicity. In this paper, a highly sensitive organic field‐effect transistor (OFET) using the cyclopentadithiophene‐benzothiadiazole (CDT‐BTZ) copolymers as a semiconductor is presented for easy and selective detection of different families of structural isomers, as well as between different isomers within each family. High accuracy discrimination is achieved over a range of concentrations using only a single sensing parameter derived from the OFET characteristic transfer curve. As a reference, other homopolymer‐ and donor–acceptor copolymer‐based OFET sensors are examined but do not have an equivalent sensing performance to that of the CDT‐BTZ‐based OFETs. Investigating the link between isomer absorption and swelling, supramolecular order and energy levels of the active layer reveals a unique effect of each isomer on the energy bands of the semiconducting polymer.     

Characterization of the spatial distribution of irradiance and spectrum in concentrating photovoltaic systems and their effect on multi‐junction solar cells

The irradiance and spectral distribution cast on the cell by a concentrating photovoltaic system, typically made up of a primary Fresnel lens and a secondary stage optical element, is dependent on many factors, and these distributions in turn influence the electrical performance of the cell. In this paper, the effect of spatial and spectral non‐uniform irradiance distribution on multi‐junction solar cell performance was analyzed using an integrated approach. Irradiance and spectral distributions were obtained by means of ray‐tracing simulation and by direct imaging at a range of cell‐to‐lens distances. At the same positions, I–V curves were measured and compared in order to evaluate non‐uniformity effects on cell performance. The procedure was applied to three different optical systems comprised a Fresnel lens with a secondary optical element consisting of either a pyramid, a dome, or a bare cell. Copyright © 2011 John Wiley & Sons, Ltd.     

Influence of doping on the dynamical properties of III–V–N by Raman scattering, infrared absorption and model calculations

In the framework of a rigid-ion model and using realistic Greens function theory, we have studied systematically the influence of doping on the lattice dynamics of III–V-nitrides. Our theoretical analysis is based on a sound physical appeal for the perturbation caused by different dopants in compound semiconductors. After analyzing the dynamical behavior for two sets of impurities with closest masses occupying Ga- and As-sites in GaAs, we have proposed simple empirical relationships providing corrections to the impurity–host interactions for isoelectronic defects and for dopants carrying static charges. Despite the simplicity of perturbation models, our Greens function theory provided results in good agreement with the recent Fourier-transform infrared (FTIR) absorption data for the magnitude and splitting of N-isotopic shifts of localized vibrational modes in GaAs. With high-resolution FTIR spectroscopy, we strongly believe that the ¹⁴N_As-related broad local mode (471 cm⁻¹) in GaAs can be resolved into fine structures due to Ga-host isotopes. Theoretical results for the in-band modes of As_N in cubic gallium nitride are compared and discussed with the existing Raman data on molecular beam epitaxy (MBE) grown GaN layers on GaAs substrates, as well as on GaN samples intentionally doped with As.     

Effect of Mesoscale Crystalline Structure on the Field‐Effect Mobility of Regioregular Poly(3‐hexyl thiophene) in Thin‐Film Transistors

Regioregular poly(3‐hexyl thiophene) (RR P3HT) is drop‐cast to fabricate field‐effect transistor (FET) devices from different solvents with different boiling points and solubilities for RR P3HT, such as methylene chloride, toluene, tetrahydrofuran, and chloroform. A Petri dish is used to cover the solution, and it takes less than 30 min for the solvents to evaporate at room temperature. The mesoscale crystalline morphology of RR P3HT thin films can be manipulated from well‐dispersed nanofibrils to well‐developed spherulites by changing solution processing conditions. The morphological correlation with the charge‐carrier mobility in RR P3HT thin‐film transistor (TFT) devices is investigated. The TFT devices show charge‐carrier mobilities in the range of 10^–4 ～ 10^–2 cm² V^–1 s^–1 depending on the solvent used, although grazing‐incidence X‐ray diffraction (GIXD) reveals that all films develop the same π–π‐stacking orientation, where the <100>‐axis is normal to the polymer films. By combining results from atomic force microscopy (AFM) and GIXD, it is found that the morphological connectivity of crystalline nanofibrils and the <100>‐axis orientation distribution of the π–π‐stacking plane with respect to the film normal play important roles on the charge‐carrier mobility of RR P3HT for TFT applications.     

Analysis of diversity schemes employing antenna selection in the presence of channel estimation errors and feedback delay

The main objective of this paper is to provide an extensive and complete examination on the effect of practical impairments such as channel estimation errors (CEEs) and feedback delay (FD) on the performance of diversity schemes over Nakagami‐m fading channels. Under erroneous channel estimation and outdated feedback cases, statistical expressions and several performance metrics related to the post‐processing signal‐to‐noise ratio (SNR) are derived for four different diversity schemes: transmit antenna selection (TAS)/orthogonal space–time block coding, TAS/maximal‐ratio transmission (MRT), MRT/receive antenna selection (RAS), and joint transmit and RAS. Exact analytical expressions for outage probability and average error rates of M‐ary modulations are derived in order to provide insightful perspectives on the capacity and error performance of diversity schemes that experience both CEE and FD. The asymptotic diversity order of the investigated diversity schemes are derived via a high‐SNR approximation approach. In order to assess the real‐world performance of the investigated diversity schemes and to observe their robustness or sensitivities in practical imperfections, various configurations are considered together with several performance comparisons. Also, Monte Carlo simulations are performed in order to validate the theoretical results. Copyright © 2014 John Wiley & Sons, Ltd.