Minority carrier lifetimes in HgCdTe alloys

We present the results of full band structure calculations of Fermi levels, intrinsic carrier densities, and one-photon absorption coefficients in undoped HgCdTe alloys. The full band structure is used in the calculation of majority and minority carrier densities and the minority carrier lifetimes limited by radiative, Auger-1, and Auger-7 mechanisms in both n- and p-doped alloys. The lifetimes we predicted differ substantially from those calculated with widely used analytical expressions, except in cases where the hole density is small. This difference originates from the significantly non-parabolic and anisotropic valence bands, which are of increasing significance as the hole density increases. From a comparison of the calculated and measured lifetimes, we deduce that the lifetimes at low temperatures are limited by the Shockley-Read-Hall (SRH) recombination. We have generalized the SRH expression to include Fermi-Dirac statistics, but we still treat the density, energy level, and cross section as adjustable parameters. We find that the calculated radiative and Auger recombination lifetimes, as well as SRH lifetimes, can fit the measured lifetimes using traps located (a) near the conduction band edge in n-HgCdTe and (b) near the valence band edge for p-HgCdTe. In addition, the movement of Fermi level with respect to the trap level explains the observed temperature-dependence of the lifetimes. We conclude that there is considerable room for improvement in HgCdTe material quality.     

Determination of the Photon Lifetime for DFB Lasers

The photon lifetime of the distributed feedback (DFB) laser is obtained through the relations between cavity photon flux and power associated with the electric fields. The solution of the coupled wave equations provides the propagation parameters and the amplitude gain coefficients for which numerical and approximate solutions are obtained. Using the photon lifetime the threshold condition of the DFB laser within a fixed mirror cavity is determined. The evaluation of both Fabry-Peacuterot and DFB modes is shown by the calculation, using two different photon lifetimes, of all modal concentrations, with the Fermi energy as the independent variable. The current density is also evaluated using the Fermi energy and threshold current is identified as the value when the Fermi energy clamps     

Model for minority carrier lifetimes in doped HgCdTe

We calculate the radiative, Auger, and the Shockley-Read-Hall recombination rates with Fermi-Dirac statistics and accurate band structures to explain the measured temperature dependence and doping dependence of minority carrier lifetimes in three n- and one p-type sample. We show that a trap state tracking the conduction band edge with very small activation energy can explain the lifetimes in the n-doped samples considered here. Similarly, for moderately p-doped HgCdTe alloy, a trap level at 75 meV is needed to explain the observed lifetimes. In either case, movement of Fermi level with respect to the trap level explains the temperature dependence of the lifetimes.     

The role of the AlGaAs doping level on the optical gain of two-dimensional electron gas photodetectors

We compare the optical response of two modulation-doped heterojunction photodetectors with identical growth structure, differing only in the doping level of the wide band material. The larger photoresponse of the higher doped device cannot be explained as photocuonductive gain because neither the transit time nor the recombination lifetimes are substantially altered by the change in doping. Alternatively, we propose to explain the results on the basis of an optical gating effect. It is shown that proper control of the two-dimensional electron gas (2-DEG) channel population in dark conditions allows one to optimize optical responsivity and external quantum efficiency.     

分子和离子的荧光寿命测量和研究

荧光寿命是一种特征于分子的光化学和光物理性质的光谱参数。本工作报道一种实验装置和技术可测定～20纳秒至数毫秒范围的荧光寿命,并对有机化合物、稀土螯合物、发光材料等不同类型样品的寿命数值大小与它们的分子结构、能级跃迁和光化学行为的关联作了讨论。     

A novel numerical technique for solving the one-dimensionalSchroedinger equation using matrix approach-application to quantum wellstructures

A numerical technique that allows straightforward determination of bound-state and quasi-bound-state energy eigenvalues (and lifetimes of the latter) for arbitrary one-dimensional potentials is presented. The method involves straightforward multiplication of 2×2 matrices and does not involve any iterations. The applicability of the technique to analysis of the quantum-well structures is also shown. Since the Schroedinger equation for a spherically symmetric potential can be transformed to a one-dimensional equation, all such problems can also be solved using this method     

Emerging techniques for long lived wireless sensor networks

In recent years, sensor networks have transitioned from being objects of academic research interest to a technology that is frequently being deployed in real-life applications and rapidly being commercialized. However, energy consumption continues to remain a barrier challenge in many sensor network applications that require long lifetimes. Battery-operated sensor nodes have limited energy storage capability due to small form-factors, or operate in environments that rule out frequent energy replenishment, resulting in a mismatch between the available energy budget for system operation and the required energy budget to obtain desired lifetimes. This article surveys several techniques that show promise in addressing and alleviating this energy consumption challenge. In addition to describing recent advances in energy-aware platforms for information processing and communication protocols for sensor collaboration, the article also looks at emerging, hitherto largely unexplored techniques, such as the use of environmental energy harvesting and the optimization of the energy consumed during sensing.     

Measuring and Modeling the Power Consumption of Energy-Efficient FPGA Coprocessors for GEMM and FFT

In this paper we analyze the power consumption and energy efficiency of general matrix-matrix multiplication (GEMM) and Fast Fourier Transform (FFT) implemented as streaming applications for an FPGA-based coprocessor card. The power consumption is measured with internal voltage sensors and the power draw is broken down onto the systems components in order to classify the energy consumed by the processor cores, the memory, the I/O links and the FPGA card. We present an abstract model that allows for estimating the power consumption of FPGA accelerators on the system level and validate the model using the measured kernels. The performance and energy consumption is compared against optimized multi-threaded software running on the POWER7 host CPUs. Our experimental results show that the accelerator can improve the energy efficiency by an order of magnitude when the computations can be undertaken in a fixed point format. Using floating point data, the gain in energy-efficiency was measured as up to 30 % for the double precision GEMM accelerator and up to 5 × for a 1k complex FFT.     

Query-enabled sensor networks using the cyclic symmetric wakeup design

The problem of conserving energy under multiple constraints of query waiting delays and sensing coverage preservation in query-based wireless sensor networks is addressed. We propose a network architecture based on the cyclic symmetric block design class of wakeup schemes. Using a target tracking application as an example, specific requirements of the application translates to design parameters in our proposed solution. Our proposal demonstrates good delay performances, high target tracking accuracies and low target identification errors, without the need for costly double-radio channels while increasing network lifetimes by a factor of four to eight over traditional methods.     

Modeling the human knee for assistive technologies

In this paper, we use motion capture technology together with an EMG-driven musculoskeletal model of the knee joint to predict muscle behavior during human dynamic movements. We propose a muscle model based on infinitely stiff tendons and show this allows speeding up 250?times the computation of muscle force and the resulting joint moment calculation with no loss of accuracy with respect to the previously developed elastic-tendon model. We then integrate our previously developed method for the estimation of 3-D musculotendon kinematics in the proposed EMG-driven model. This new code enabled the creation of a standalone EMG-driven model that was implemented and run on an embedded system for applications in assistive technologies such as myoelectrically controlled prostheses and orthoses.     

Effect of coulomb correlations on luminescence and absorption in compensated semiconductors

The spectra of donor–acceptor light absorption and luminescence in lightly doped and lightly compensated semiconductors are calculated. In the photoluminescence calculation, two limiting cases of long and short carrier lifetimes relative to the carrier-energy relaxation time are considered. It is shown that, at long lifetimes, the photoluminescence spectrum is significantly shifted toward longer wavelengths due to the relaxation of minority charge carriers. At intermediate lifetimes, the photoluminescence spectrum consists of two peaks, which is in good agreement with the experimental data.     

On the spectra of electrons and holes in an open spherical nanoheterostructure (through the example of GaAs/AlxGa1−x As/GaAs)

The spectra of the quasi-stationary electron and hole states in an open spherical nanoheterostructure are calculated in the effective mass approximation using the S-matrix theory. Numerical calculation is performed through the example of the GaAs/Al_xGa_1?x As/GaAs system. A complete system of electron and hole wave functions is obtained for nanoheterostructures consisting of an arbitrary number of layers. Quasiparticle lifetimes as a function of the GaAs/Al_xGa_1?x As/GaAs nanosystem geometrical parameters and Al content x are calculated.     

The effect of single-phonon and plasmon recombination on the lifetime in n-Hg1−xCdxTe with magnetically tuned bandgap

Theoretical investigations have predicted a great enhancement of electron-hole recombinations by adjusting the bandgap of narrow-gap semiconductors to the energy of elementary excitation in solids such as phonons or plasmons. Such an enhancement of the recombination rate of excess carriers is very important in constructing far-I.R. devices and might be used to generate high densities of LO-phonons and plasmons. We report experimental evidence of the influence of these recombination channels on the photoconductivity and the excess carrier lifetime in semimetallic n-Hg_1−xCd_xTe alloys, whose induced bandgap is magnetically tuned through the relevant energies of the elementary excitations. Both from a comparison of theoretically estimated lifetimes with Auger lifetimes and from experimental observations the new recombination channels prove to be very efficient and dominate the behaviour near their resonance with the band-gap.     

A distributed activity scheduling algorithm for wireless sensor networks with partial coverage

One of the most important design objectives in wireless sensor networks (WSN) is minimizing the energy consumption since these networks are expected to operate in harsh conditions where the recharging of batteries is impractical, if not impossible. The sleep scheduling mechanism allows sensors to sleep intermittently in order to reduce energy consumption and extend network lifetime. In applications where 100% coverage of the network field is not crucial, allowing the coverage to drop below full coverage while keeping above a predetermined threshold, i.e., partial coverage, can further increase the network lifetime. In this paper, we develop the distributed adaptive sleep scheduling algorithm (DASSA) for WSNs with partial coverage. DASSA does not require location information of sensors while maintaining connectivity and satisfying a user defined coverage target. In DASSA, nodes use the residual energy levels and feedback from the sink for scheduling the activity of their neighbors. This feedback mechanism reduces the randomness in scheduling that would otherwise occur due to the absence of location information. The performance of DASSA is compared with an integer linear programming (ILP) based centralized sleep scheduling algorithm (CSSA), which is devised to find the maximum number of rounds the network can survive assuming that the location information of all sensors is available. DASSA is also compared with the decentralized DGT algorithm. DASSA attains network lifetimes up to 92% of the centralized solution and it achieves significantly longer lifetimes compared with the DGT algorithm.     

Computer-aided study of steady-state carrier lifetimes under arbitrary injection conditions

The steady-state minority and majority carrier lifetimes are calculated using an exact steady-state equivalent circuit model. The exact majority and minority carrier lifetimes are calculated as functions of position in a diffused silicon P/N junction diode doped with zinc, and as functions of bias voltage or injection level. Factors which affect carrier lifetimes are pointed out and illustrated. Lifetimes due to the presence of multiple energy level Shockley-Read-Hall centers are also discussed. The exact carrier lifetimes at very low and very high injection levels computed from this model agree well with those calculated from the analytical expressions derived by Shockley and Read.     

Finite-difference time-domain calculation of the spontaneousemission coupling factor in optical microcavities

We present a general method for the β factor calculation in optical microcavities. The analysis is based on the classical model for atomic transitions in a semiconductor active medium. The finite-difference time-domain method is used to evolve the electromagnetic fields of the system and calculate the total radiated energy, as well as the energy radiated into the mode of interest. We analyze the microdisk laser and compare our result with the previous theoretical and experimental analyses. We also calculate the β factor of the microcavity based on a two-dimensional (2-D) photonic crystal in an optically thin dielectric slab. From the β calculations, we are able to estimate the coupling to radiation modes in both the microdisk and the 2-D photonic crystal cavity, thereby showing the effectiveness of the photonic crystal in suppressing in-plane radiation modes     

Characterization of the heat loading of Nd-doped YAG, YOS, YLF, andGGG excited at diode pumping wavelengths

The parameter ξ (xi) is proposed as an alternative to the traditional solid-state laser media heating parameter, χ (chi). ξ is the ratio of heat produced to energy absorbed, and χ is the ratio of heat produced to the maximum stored energy in the upper laser level. The parameter ξ is particularly relevant to diode pumped systems. We demonstrate an experimental ξ characterization based on the determination of the steady state cooling rate (hence heating rate) of small sample crystals subjected to pump laser heating. Using measured fluorescent lifetimes of the samples and near zero doping (intrinsic) values, the doping independent (zero doping or zero quenching) parameters χφ and ξφ are determined. The results for all samples are in excellent agreement with calculations based solely on energy defect and nonradiative quenching of the upper level     

Bound and quasibound state calculations for biased/unbiasedsemiconductor quantum heterostructures

A complex transcendental equation valid for a wide range of electron energies for semiconductor quantum heterostructures under unbiased or biased conditions is derived. Its complex roots have as real parts the structure eigenenergy levels, and their imaginary parts are directly related to the lifetime of the corresponding eigenenergies. A numerical method is presented that is capable of extracting all these complex eigenenergies. The method is based on the argument principle theorem from complex number theory. Therefore, all the energy levels and lifetimes of bound and quasibound states can be determined. Energy levels and lifetimes can also be calculated in the presence of scattering events when these are modeled with an energy broadening imaginary potential. Extensive comparisons between this numerical method and other currently used techniques are included, proving the generality and the accuracy of this new method     

Analysis of the optical gain characteristics of semiconductor quantum-dash materials including the band structure modifications due to the wetting layer

We present a numerical model for the calculation of the opto-electronic properties of a semiconductor InAs-InAlGaAs quantum dash active material including the presence of the wetting layer (WL), formed at the bottom of the dashes, and the quantum mechanical coupling among dashes caused by the high density of the InAs islands. The model calculates self-consistently the conduction and valence band energy diagram of the confined and unconfined states, the corresponding density of states, the electron and hole wavefunctions and the gain spectra. The results obtained are also compared with a more simple model that consider dashes as isolated and without the WL. The comparison evidences the role of the WL in limiting the gain performance such as the maximum gain, the differential gain and the optical gain bandwidth. The numerical tool is then used to design an improved quantum dash material, which allows to overcome these gain limitations even in presence of the WL and the high dash density.