Dependence of high-speed properties on the number of quantum wellsin 1.55 μm InGaAs-InGaAsP MQW λ/4-shifted DFB lasers

The dependence of intrinsic dynamic properties, such as relaxation oscillation frequency, damping K-factor, and spectral chirping under 10-Gb/s direct modulation, on the number of quantum wells is systematically investigated in 1.55-μm multiquantum-well λ/4-shifted distributed-feedback lasers. The theoretical and experimental results indicate that the dependence of the above three factors on the number of quantum wells is clearly explained by the linear gain saturation of the quantum-well lasers     

多量子阱有效折射率偏振相关性研究

分析了多量子阱材料各参数对其TE模和TM模有效折射率的影响。结果表明:阱数增多,多量子阱有效折射率降低,当量子阱数目大于3时,其有效折射率的变化不明显。垒厚增加,有效折射率略有降低。存在合适的张应变量使TE模和TM模有效折射率峰值波长接近的同时,折射率差值整体最小,偏振相关性最小。据此提出多量子阱材料有效折射率低偏振相关设计方法,并设计出C波段内(1530~1565 nm)折射率低偏振相关的InGaAs/InGaAsP多量子阱材料。研究结果有助于设计实用化的有效折射率低偏振相关量子阱材料。     

EBIC characterization of light-emitting structures based on GaN

EBIC investigations of light-emitting structures based on InGaN/GaN MQW with different numbers of wells have been carried out. A pronounced dependence of collection efficiency on quantum-well number is observed. A comparison with apparent carrier profiles calculated from C-V curves reveals a correlation between the collection efficiency and location of quantum well inside the depletion region. Defects producing bright EBIC contrast are revealed in the structure with five quantum wells. This contrast is associated with defects locally decreasing the excess carrier recombination inside quantum wells. The text was submitted by the authors in English.     

Characterization of MOVPE-grown (Al, In, Ga) N heterostructures by quantitative analytical electron microscopy

The low pressure metalorganic vapor phase epitaxy growth of wurzite (Al, In, Ga)N heterostructures on sapphire substrates is investigated by quantitative analytical scanning transmission electron microscopy techniques like atomic number (Z-) contrast imaging and convergent beam electron diffraction (CBED). Especially (In, Ga)N quantum wells of different thicknesses as well as superlattices were analyzed with respect to defects, chemical composition variations, interface abruptness and strain (relaxation) effects. The interfaces in In_0.12Ga_0.88N/GaN quantum wells appear to be asymmetric. Additionally, we found composition variations of Δx_In≥0.03 within the InGaN quantum wells. The application of electron diffraction techniques (CBED) yields quantitative information on strain and relaxation effects. For the case of 17 nm thick InGaN quantum wells, we observed relaxation effects which are not present in the investigated thin quantum wells of 2 nm thickness. The experimentally obtained diffraction patterns were compared to simulations in order to get values for strain within the quantum wells. Additionally, the influence of dislocations on the digression of superlattices is investigated.     

Constraints on coherent control of quantum-well excitons forhigh-speed all-optical switching

Coherent control of excitons in quantum wells at high repetition rates is investigated theoretically to determine the practical constraints for application in high-bit-rate optical switching. A tradeoff between optical pathlength (number of quantum wells) and excitation density is found that severely limits the applicability of this mechanism for implementation in 100-Gb/s optical systems     

Effects of well number, cavity length, and facet reflectivity onthe reduction of threshold current of GaAs/AlGaAs multiquantum welllasers

The optimum design for reducing the threshold current of GaAs/AlGaAs multiquantum well lasers is determined experimentally. The lowest threshold current density is realized by using single and multiquantum wells at long and short cavity lengths, respectively. The threshold current has a minimum at the optimum cavity length: the minimum threshold current is smaller for a larger number of quantum wells, and the optimum cavity length is inversely proportional to the number of wells. Experiments are compared to the theory developed by P.W.A. McIlroy, et al. (ibid., vol.21, no.12, p.1958-63, 1985) and limiting performances of quantum well lasers with various numbers of wells are presented. The reduction of the threshold current by high reflectivity coatings is also demonstrated, and a threshold current as low as 1.86 mA at 15°C is reported     

Effect of active-region “volume” on the radiative properties of laser heterostructures with radiation output through the substrate

The radiative properties of InGaAs/GaAs/InGaP laser structures with radiation output through the substrate depending on the number of quantum wells in the active region and laser diodes on their basis are investigated. It is established that the presence of six–eight quantum wells in the active region is optimum from the viewpoint of observable values of the threshold current and the output optical power of lasers.     

A comparison of amplitude-phase coupling and linewidth enhancementin semiconductor quantum-well and bulk lasers

The amplitude-phase coupling factor α (linewidth enhancement factor) is compared for typical semiconductor quantum-well and bulk double heterostructure lasers. As a direct consequence of the reduction of the differential gain, there is no reduction of α in single-quantum-well lasers compared to bulk lasers. The number of quantum wells strongly affects the amplitude-phase coupling in quantum-well lasers. It is shown that the interband transition induced amplitude-phase coupling dominates that induced by the plasma effect of carriers in typical quantum-well lasers. By considering the spontaneous emission factor in the spectral linewidth, the authors show that there is an optimal number of quantum wells for achieving the narrowest spectral linewidth     

Dependence of intersubband absorption on the number of quantumwells: Radiative coupling effects

On the basis of a microscopic theory, in which the electromagnetic and electronic nonlocalities have been incorporated, dependence of the intersubband absorption on the number of quantum wells (QWs) is studied. Detailed numerical simulations show that changes in the number of wells, the angle of incidence, and the barrier thickness between adjacent wells can significantly modify the radiative coupling among QWs. Consequently, the intersubband absorption spectrum can also be changed     

Studies of physical phenomena in semiconductor nanostructures using samples with laterally nonuniform layers: Photoluminescence of tunneling-coupled quantum wells

The previously suggested spectral-correlative method for studying nanostructures is applied to an analysis of photoluminescence of tunneling-coupled and isolated quantum wells in structures with laterally nonuniform layers. This method made it possible to use a single wafer to study the dependences of intensities of photoluminescence lines and their energy positions on the tunneling-barrier width for a system of tunneling-coupled GaAs-InGaAs-GaAs wells and on the quantum-well widths in a system of isolated AlGaAs-GaAs-AlGaAs quantum wells. Good agreement between the results of calculations and experimental data can be attained if it is assumed that a constant transverse electric field affecting the processes of trapping of charge carriers by quantum wells exists in a structure with tunneling-coupled quantum wells. The dependence of photoluminescence parameters on the width of isolated quantum wells is sensitive to the profile of heterointerfaces and to the processes of trapping the charge carriers by quantum wells.     

Calculation of the propagation constant of optical modes inmultiquantum-well structures

A simple mathematical formalism is used to calculate the propagation constant β for optical modes (TE₀, TM₀ ) in multiquantum-well semiconductor laser structures. The energy correction due to narrow quantum wells, the energy dependence of the refractive indexes, and the energy dependence of electron and hole masses are taken into account. The data show that there is a considerable difference between the calculated values and the averaged values for β, especially for a small number of narrow quantum wells     

Empirical formulas for design and optimization of 1.5 μmInGaAs/InGaAsP strained-quantum-well lasers

The effects of strain and number of quantum wells on optical gain, differential gain, and nonlinear gain coefficient in 1.55-μm InGaAs/InGaAsP strained-quantum-well lasers are theoretically investigated. Well-approximated empirical expressions are proposed to model these effects. Using these formulas, one can easily and accurately predict the performance of a laser diode for a given structure. Therefore, these empirical formulas are useful tools for design and optimization of strained quantum well lasers. As a general design guideline revealed from the empirical formulas, the threshold current is reduced with the compressive strain, and the modulation bandwidth is most efficiently increased with the number of wells     

Temperature-dependent excitonic absorption in long-period multiple In<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>As/GaAs quantum well structures

Temperature variations in the fundamental absorption edge of long-period In _x Ga_{1 − x} As/GaAs structures are studied for samples with different numbers of quantum wells and similar periods. The quantum wells were close in composition and width. Experimental data are interpreted in the model of exciton-polariton light transfer involving localized excitons in confined structures with a finite number of quantum wells. The experimentally observed low-temperature anomaly of the integrated absorption coefficient is attributed to reemission of resonance localized excitons along a finite chain of quantum wells, with no excitonic transfer. The radiative decay time of an exciton in a single quantum well is estimated from the experimental data. It is demonstrated that, at low temperatures, the major contribution to the width of the experimentally observed absorption line corresponding to the ground heavy-hole exciton state is made by inhomogeneous broadening of the line by the field of potential fluctuations associated with the compositional disorder of the alloy. At low temperatures, the inhomogeneous broadening is much more pronounced than the broadening governed by the true radiative and nonradiative dissipative decay.     

量子阱间耦合对带结构的影响

用Matlab程序语言数值计算了多量子阱结构的能级,并研究了量子阱间耦合对其带结构的影响。多量子阱模型是在一个一维有限深势阱（阱边势垒高为 V0）中插入等高（Vb）等厚（b）的势垒方式建成,被分割而成的多量子阱厚度为w。通过增加插入壁垒的个数N、改变阱垒厚度比w/b 及势垒高度比V0/Vb,分别近似计算了对应的结构的能级及波函数。计算结果显示,量子阱间耦合受上述参数的强烈影响,改变参数N,w/b 或V0/Vb ,能带和带隙的宽度是可以被调节的。研究说明,量子阱的能带及带隙宽度达到期望值是完全有可能实现的。     

Electric field effects in AlGaAs-GaAs symmetric and asymmetriccoupled quantum wells

A theoretical study of energy level shift and field-induced tunneling in symmetric and asymmetric coupled quantum wells is presented. Energy level shift is calculated from the time-dependent Schrodinger equation using the inverse power method. The time evolution of an electron wavepackage is shown by the application of the time-development operator of the time-independent Schrodinger equation. Energy level shift in coupled quantum wells is found to be enhanced in comparison to single quantum wells. The energy level shift in coupled quantum wells is found to be nearly linear with the applied field. Oscillation frequencies for electrons in symmetric and asymmetric coupled quantum wells are evaluated versus the applied field and compared to semiclassical prediction. Tunneling lifetimes in symmetric and asymmetric coupled quantum wells are evaluated versus the applied field     

发光量子阱对波长上转换红外探测器效率的影响

波长上转换红外探测器在实现大面阵、低暗电流红外探测方面具有很大的发展潜力.短波长光子的发光效率是影响上转换器件效率的重要因素.设计、制作了具有不同发光阱个数的波长上转换红外探测器件,结合器件的红外响应和仿真计算,分析了发光阱个数对波长上转换效率的影响规律.研究结果表明,选择单个发光阱,有利于提高器件的发光效率,从而提高波长上转换效率.     

Low-Temperature photoluminescence of SiGe/Si disordered multiple quantum wells and quantum well wires

Low-temperature photoluminescence from disordered SiGe/Si quantum wells and quantum wires made from periodic quantum wells by electron beam lithography and reactive ion etching has been measured. No enhancement in luminescence is seen, compared to that in periodic quantum wells, in the disordered wells or quantum wires. New transitions are observed in the wire luminescence, including a possible no-phonon transition exhibiting a 32 meV blue shift compared to the same transition in the wells.     

Molecular beam epitaxy growth of InGaSb/AlGaAsSb strained quantum well diode lasers

2μm InGaSb/AlGaAsSb strained quantum wells and a tellurium-doped GaSb buffer layer were grown by molecular beam epitaxy（MBE）.The growth parameters of strained quantum wells were optimized by AFM, XRD and PL at 77 K.The optimal growth temperature of quantum wells is 440℃.The PL peak wavelength of quantum wells at 300 K is 1.98μm,and the FWHM is 115 nm.Tellurium-doped GaSb buffer layers were optimized by Hall measurement.The optimal doping concentration is 1.127×10¹⁸ cm^-3 and the resistivity is 5.295×10^-3Ω·cm.     

ZnSe／ZnS多量子阱光开关的优化设计

提出了ZnSe/ZnS多量子阱光开关的综合优化设计方案。根据Kronig-Penney模型对多量子阱结构进行优化,根据实验结果对多量子阱构成的非线性F-P进行了优化设计。     

Evolution of the deformation state and composition as a result of changes in the number of quantum wells in multilayered InGaN/GaN structures

The methods of high-resolution X-ray diffraction have been used to study the multilayered structures in an In _x Ga_{1 − x} N/GaN system grown by the method of metal-organic chemical-vapor deposition. A correlation between the strain state (relaxation) of the system, the indium content within quantum wells, the ratio of the barrier/well thicknesses, and the number of quantum wells in the active superlattice is established. It is shown that partial relaxation is observed even in a structure with one quantum well. The results we obtained indicate that the relaxation processes are bound to appreciably affect the optical characteristics of devices.