Hot-carrier dynamics in Ge on single picosecond timescales: Comparing Raman and reflectivity experiments with a self-consistent kinetic model

Through comparisons of picosecond Raman and transient reflection experiments with a comprehensive kinetic model of photo-excited carrier and lattice dynamics in Ge, we demonstrate the ability of these techniques to probe subtle aspects of non-equilibrium carrier dynamics in group IV materials at moderate injected carrier densities. Using material parameters taken from the literature, the temporal evolution of the non-equilibrium optical phonon population generated by the relaxation of photo-excited electrons and holes is obtained by solving a coupled set of Boltzmann equations for the electron and hole particle and thermal currents. The results of the calculation agree, in absolute terms, with the experimentally observed evolution of the non-equilibrium optical phonon population. The calculation also predicts that the hot plasma initially diffuses rapidly away from the sample surface, on a 5 picosecond timescale, and subsequently diffuses much slower as the carrier temperature decays to the lattice temperature, and the density gradient diminishes due to the hot carriers which have already migrated into the material. This prediction is verified by comparison of the calculated change in reflectivity due to the plasma, and picosecond reflectivity measurements performed at room temperature with 575 nm pulses.     

Hot phonon effects in ZnSe

Femtosecond pump-probe differential reflectivity spectroscopy is used to investigate the ultrafast cooling dynamics of hot photoexcited carriers in a high-quality ZnSe epilayer grown on GaAs. Comparison with a theoretical model based on a balance equation approach indicates that the observed reduction in the electron cooling rate with increasing carrier density (for carrier densities greater than ～3×10¹⁷cm^?3) is due to both screening of the Fröhlich interaction and a non-equilibrium hot longitudinal optic-phonon population generated by the cooling electron distribution.     

MINIMOS 3: A MOSFET simulator that includes energy balance

We present a model for hot carrier transport which is implemented in the device simulator MINIMOS 3. A brief resume of the model is given. We present various results which were calculated with this new model. We show that the I-V characteristics of a MOSFET can be calculated from Leff= 10 µm down to Leff= 0.9 µm with one parameter set. Modifications of carrier and current distributions are presented that show how hot carrier effects tend to smooth these distributions. Implications are discussed how a self-consistent carrier temperature can be used to model impact ionization and oxide injection.     

Analytical model of a superluminescent diode

Using a set of traveling wave rate equations ,a superluminescent diode with a low facet reflectivity is studied .Analytical expressions of the distribu-tions of carrier density ,forward-and backward-propagation photon densities,and gain are obtained at different facet reflectivities.It is shown that the high nonuni-form carrier distribution is evident in the case of low facet reflectivity.The results can serve as useful guides in understanding emission mechanism of superlumi-nescent diodes.     

Analytical model of a superluminescent diode

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Silicon surface emission of hot electrons

A new emission model, based on a probabilistic treatment of electron trajectories, is developed for hot electron emission from a silicon surface. Primary electrons, generated thermally or optically, are heated by a normal electric field and cooled by phonon collisions and by impact ionization. Unlike the case in previous models, the semiconductor field need not be uniform, and multistage phonon processes are included. The model provides hot electron energy distributions both for electrons transported in the non-equilibrium layer at the silicon surface and for electrons emitted into the oxide. It shows that the most probable trajectory of an emitted electron is not one of zero collisions (as assumed in the analysis of Verwey et al. [2]), but one involving the generation of many optical phonons. The model, used together with photoexcited hot electron measurements (as developed by Ning and Yu [6]), also provides an accurate method for determining phonon and ionization mean free paths.     

Amplified spontaneous emission and carrier pinning in laser diodes

Theoretical and experimental results for the temperature dependence of amplified spontaneous emission (ASE) in laser diodes (LDs) and light-emitting diodes (LEDs) are presented. The theoretical model takes into account conduction band nonparabolicity and band-gap renormalization. The gain spectrum is calculated from the theoretical spontaneous emission spectrum, and both compare very well with experimental data. From a fit to the observed temperature dependence of ASE for an LED and the gain spectrum for an LD with a structure identical to that of the LED except for mirror reflectivity, it is possible to establish carrier density as a function of injection current for both devices. It is shown that photons fluctuating into cavity modes give rise to substantial subthreshold carrier pinning in laser diodes. These fluctuations extract an extra current from the device and play an increasingly important role with increasing temperature     

Electron transport and ionization in silicon at high fields

The saturated drift velocity measured for electrons at high fields is inconsistent with Shockley's model for impact ionization in silicon. It is explained in terms of a field-dependent mean free path for high energy phonon creation in the electric field direction, electrons creating a high energy phonon as soon as they have acquired sufficient energy from the field. Assuming that the electron wavepacket travels at the saturated drift velocity without dispersion, it can be shown that the increased scattering rate at high fields must result in a large spread in the carrier energy. If a drifted Maxwellian distribution is assumed, a unique expression can be obtained for the carrier temperature T^* which is in good agreement with the measured field dependence of the ionization coefficient. In this model, a cylindrical hot carrier distribution must be assumed with the hot carrier energy in a plane perpendicular to the applied field. Exact calculations of the magneto-resistance of such a distribution can be made verifying that the drift velocity is indeed saturated.     

Numerical Analysis of Gain Saturation, Noise Figure, and Carrier Distribution for Quantum-Dot Semiconductor-Optical Amplifiers

The gain saturation behaviors and noise figure are numerically analyzed for quantum-dot semiconductor optical amplifiers (QD-SOAs). The carrier and photon distributions in the longitudinal direction as well as the photon energy dependent facet reflectivity are accounted in the rate equations, which are solved with output amplified spontaneous emission spectrum as iterative variables. The longitudinal distributions of the occupation probabilities and spectral-hole burning are presented for electrons in the excited and ground states of quantum dots. The saturation output power 19.7 dBm and device gain 20.6 dB are obtained for a QD-SOA with the cavity length of 6 mm at the bias current of 500 mA. The influences of the electron intradot relaxation time and the QD capture time on the gain spectrum are simulated with the relaxation time of 1, 30, and 60 ps and capture time of 1, 5, and 10 ps. The noise figure as low as 3.5 dB is expected due to the strong polarization sensitive spontaneous emission. The characteristics of gain saturation and noise figure versus input signal power for QD-SOAs are similar to that of semiconductor linear optical amplifiers with gain clamping by vertical laser fields.     

Low density photoexcitation phenomena in semiconductors: Aspects of theory and experiment

The basic mechanisms related to the photoexcitation of electron-hole pairs in semiconductors under conditions of low excitation density, low temperature and high crystal purity are reviewed. The use of high-resolution emission spectroscopy of band-to-impurity optical transitions in GaAs to measure the energy distribution functions $\text{?(E)}$ of electrons and holes in optically excited carrier plasmas of well defined densities (10¹⁰ cm^?3≤n≤ 10¹³ cm^?3) is described. With this experimental method (i) the energy relaxation of initially hot carrier distributions after pulsed photoexcitation ($\text{h}\text{?}\text{ω ? E}{}_{\mathrm{g}}$), (ii) stationary non-equilibrium distributions of electrons in the conduction band under cw photoexcitation ($\text{h}\text{?}\text{ω ? E}{}_{\mathrm{g}}$) and (iii) the transport properties of resonantly excited carrier plasmas in low electric fields ($\text{0≤|}\text{F}\text{|≤20}\text{V/cm}$) are investigated. The observed distribution functions are compared with theoretical results on the basis of the known band structure data of GaAs, taking into account polar optic and acoustic phonon scattering, the interaction among the carriers, ionized impurity scattering, and using approximate solutions of the appropriate transport equation.     

激光相位调制法布里-珀罗腔精细度法测定反射率

谐振腔精细度法是通过测量光学谐振腔透射谱线宽度来实现对光学反射镜反射率的测定。由于此谱线宽度数值通常在射频范围,采用光谱仪难以精确测量,而在谐振腔精细度方法的基础上引入激光相位调制技术,提出激光相位调制法布里-珀罗(F-P)腔精细度法测定反射率。利用电光调制器对激光进行射频相位调制,以产生的调制边带与激光载波的频率间隔作为“射频标尺”,精确测量了法布里-珀罗腔透射光谱的谱线宽度。利用腔精细度与腔透射光谱的谱线宽度及反射率的关系公式,获得了反射镜的反射率,测量精度可达到10-4。     

降雨的雷达体反射率研究

利用点匹配法计算了不同雨滴谱分布的降雨雷达体反射率，结果表明M—P分布与我国实测雨滴谱的结果比较接近，同时进一步分析了降雨雷达体反射率的频率、极化特性等。根据计算结果，研究得到了体反射率的新解析公式，其与点匹配法计算的结果有较好的一致性。     

单晶硅表面载流子动力学的超快抽运探测

利用800 nm波长的飞秒抽运探测技术测量了单晶硅表面50 ps内的瞬态反射率变化,研究了表面载流子的超快动力学过程.基于自由载流子密度变化过程建立的反射率模型可以很好地描述瞬态反射率变化,说明受激自由载流子超快响应的贡献主导了反射率的变化过程.经拟合获得了样品的表面复合速度(SRV)为1.2×106cm/s.建立了耦合的载流子输运模型,探讨了单晶硅表面热载流子的密度、温度随时间的演化过程.研究表明,表面复合过程是影响本征单晶硅表面载流子动力学的重要因素.     

端面反射率对单端反射半导体光放大器的影响

本文考虑了光子密度、增益谱色散、放大自发辐射噪声(ASE)、端面反射率和载流子浓度对折射率的影响,并采用细化分段法及四阶龙格-库塔法求解速率方程及光场传输方程,分析了不同端面反射率时单端反射半导体光放大器(RSOA)的动态特性。结果表明端面反射率的适当增加,可以使增益得到提高,增益恢复时间减少, ASE输出功率增加,消光比下降。理论分析为优化RSOA的动态特性具有一定的指导意义。     

Arsenic/phosphorus LDD optimization by taking advantage ofphosphorus transient enhanced diffusion for high voltage input/outputCMOS devices

Optimization of a LDD doping profile to enhance hot carrier resistance in 3.3 V input/output CMOS devices has been performed by utilizing phosphorus transient enhanced diffusion (TED). Hot carrier effects in hybrid arsenic/phosphorus LDD nMOSFET's with and without TED are characterized comprehensively. Our result shows that the substrate current in a nMOSFET with phosphorus TED can be substantially reduced, as compared to the one without TED. The reason is that the TED effect can yield a more graded n^- LDD doping profile and thus a smaller lateral electric field. Further improvement of hot carrier reliability can be achieved by optimizing arsenic implant energy. Secondary ion mass spectrometry analysis for TED effect and two-dimensional (2-D) device simulation for electric field and current flow distributions have been conducted. The phosphorus TED effects on transistor driving current and off-state leakage current are also investigated     

A New Cooperative Spectrum Sensing Scheme for Cognitive Ad-Hoc Networks

As the radio spectrum is becoming more and more crowded, cognitive radio has recently become a hot research topic to improve the spectrum utilization efficiency. It is well known that the success of cognitive radio depends heavily on fast and efficient spectrum sensing that is very difficult in practice. Toward this end, this paper introduces a new guard-resident cooperative spectrum sensing scheme for a cognitive ad-hoc network. In particular, we classify cognitive nodes as either resident or guard based on the spectrum neighbor decision and distributed boundary search. The guard nodes sense the spectrum and then inform the resident nodes that are greatly relieved from spectrum sensing about the radio environmental changes. The analysis and simulation results show that the proposed algorithm can significantly reduce the total spectrum sensing load without sacrificing the sensing accuracy.     

Device characterization of 16/14 nm FinFETs for reliability assessment with infrared emission spectra

Our recent research shows that hot carrier photon emission spectra can deliver several important voltage-dependent device parameters of modern FinFET devices including electron and hole energy distributions, maximum electric field strength in the FET channel, free mean path length and temperature approximations for the hot carrier gas in the channel. These device parameters can be used to continuously monitor device degradation and to obtain estimates for carrier energies and scattering processes within the FET. Furthermore, the gate current function over voltage can be derived from the data using a suitable theoretical model, which can support reliability forecasts. The data presented here is almost impossible to obtain with other methods for an active device. PEM holds the further advantage of being non-invasive and only collects photons emitted by regular FET operation, leaving transistor functionality and the whole chip unaltered.     

Hot carrier-induced degradation of gate overlapped lightly dopeddrain (GOLDD) polysilicon TFTs

Hot-carrier injection is known to produce interface states and oxide trapped charge, which, depending upon their spatial distribution, can strongly influence the local electric fields as well as the current flow. In this work, we analyze the hot carrier-induced degradation of gate overlapped lightly doped drain (GOLDD) polysilicon thin film transistors (TFTs) and a new model, which correlates the interface state generation with the hot carrier injection current, is proposed. The defect generation rate has been assumed to depend upon the product of hot electron and hole currents J_eh, and the resulting interface state distribution has been evaluated self-consistently with the current density and carrier concentration distributions. By successive iterations, a complete spatial and time evolution of the interface state distribution has been determined, and the electrical characteristics, calculated with these interface state distributions are in good agreement with the experimental data     

Fourier-optical transverse mode selection in external-cavity broad-area lasers: experimental and numerical results

Broad-area lasers (BALs) with external Fourier-optical cavities with spatial filter for transverse mode selection have been studied experimentally and theoretically. The transverse mode structure of BALs is modeled using a three mirror cavity approach. The model accounts for gain saturation, carrier diffusion, and anti-guiding effects. Near- and far-field distributions are calculated and compared to experimental results. Transverse mode selection is achieved for BALs with a residual front facet reflectivity of 10% at low pump currents. For BALs having a very low front facet reflectivity of 0.001%, transverse modes can be selectively excited up to pump currents more than 200% above laser threshold. Calculations show that BALs having a 0.001%-antireflection coating with an external Fourier-optical cavity high above threshold can operate in a self-Q-switched-like mode with pulse durations of 2-4 ns and repetition rates of 100-200 MHz. Experimental results obtained with a hybrid integrated-optical external cavity for transverse mode selection are also presented.