Calculating the optical properties of multidimensionalheterostructures: Application to the modeling of quaternary quantum welllasers

A method for calculating the electronic states and optical properties of multidimensional semiconductor quantum structures is described. The method is applicable to heterostructures with confinement in any number of dimensions: e.g. bulk, quantum wells, quantum wires and quantum dots. It is applied here to model bulk and multiquantum well (MQW) InGaAsP active layer quaternary lasers. The band parameters of the quaternary system required for the modeling are interpolated from the available literature. We compare bulk versus MQW performance, the effects of compressive and tensile strain, room temperature versus high temperature operation and 1.3 versus 1.55 pm wavelength operation. Our model shows that: compressive strain improves MQW laser performance. MQW lasers have higher amplification per carrier and higher differential gain than bulk lasers, however, MQW performance is far from ideal because of occupation of non-lasing minibands. This results in higher carrier densities at threshold than in bulk lasers, and may nullify the advantage of MQW lasers over bulk devices for high temperature operation     

The effect of barrier composition on the vertical carrier transport and lasing properties of 1.55-/spl mu/m multiple quantum-well structures

In this paper, the effect of barrier bandgap and composition on the optical performance of 1.55-/spl mu/m InGaAsP/InGaAsP and InGaAsP/InGaAlAs multiple quantum-well structures and Fabry-Perot lasers is evaluated experimentally. Direct vertical carrier transport measurements were performed through strain-compensated multiple quantum-well (MQW) test structures using femto-second laser pulse excitation and time-resolved photoluminescence up-conversion method. MQW test structures were grown with different barrier composition (InGaAsP and InGaAlAs) and barrier bandgap (varied from /spl lambda//sub g/= 1440 to 1260 nm) having different conduction band /spl Delta/E/sub c/ and valence band discontinuity /spl Delta/E/sub v/, while keeping the same InGaAsP well composition for all the structures. The ambipolar carrier transport was found to be faster in the structures with lower valence band discontinuity /spl Delta/E/sub v/. Regrown semi-insulating buried heterostructure Fabry-Perot (SIBH-FP) lasers were fabricated from similar QWs and their static light-current-voltage characteristics (including optical gain and chirp spectra below threshold) and thermal characteristics were measured. Lasers with InGaAlAs barrier showed improved high-temperature operation, higher optical gain, higher differential gain, and lower chirp, making them suitable candidates for high-bandwidth directly modulated uncooled laser applications.     

Effect of barrier height on the uneven carrier distribution inasymmetric multiple-quantum-well InGaAsP lasers

Four asymmetric multiple-quantum-well (AMQW) laser structures have been grown and tested. The structures demonstrate that carriers are not evenly distributed across the active region of a MQW laser. Wells at the p-side of the active region are preferentially pumped indicating there are more carriers at the p-side of the active region than at the n-side. The structures also demonstrate that decreasing the height of the barriers reduces this effect and results in a more even carrier distribution. Thus, well position and barrier height are shown to be important design parameters for AMQW and conventional MQW lasers     

Novel design scheme for high-speed MQW lasers with enhanceddifferential gain and reduced carrier transport effect

We present a theoretical analysis exploring the optimum design of high-speed multiple-quantum-well (MQW) lasers for 1.55-μm operation. Various combinations of well and barrier materials are examined for lattice-matched, strained-layered (SL), and strain-compensated (SC) MQW lasers with InGaAsP and InGaAlAs barriers. The gain characteristics are investigated for these MQW lasers with various barrier bandgap wavelengths and are used to evaluate the modulation characteristics based on the carrier dynamics model which includes a set of Poisson, continuity, and rate equations. The importance of band engineering aimed at simultaneously reducing the carrier transport effect and enhancing the differential gain is described. It is shown that SC-MQW lasers with InGaAlAs barriers have an advantage in reducing the density of states in the valence band by reducing the overlap integral between the heavy- and light-hole wave functions, which effect has previously been discarded as a minor correction in designing conventional InGaAsP-based MQW lasers. Furthermore, the hole transport rate across the barriers can be drastically reduced in SC-MQW lasers due to the reduced effective barrier height for the holes. Based on this novel design scheme, a 3-dB bandwidth approaching 70 GHz is expected for 20-well SC-MQW lasers with InGaAlAs barriers as a result of both the large differential gain and reduced transport effect     

Design and performance of asymmetric waveguide nitride laser diodes

We describe the design and performance characteristics of an asymmetric waveguide nitride laser diode structure, in which the p-cladding layer is placed immediately over the multiple-quantum-well (MQW) active region. Its close proximity to the active region enables it to serve not only as a cladding layer, but also as a potential barrier that confines injected electrons. This structure represents a departure from conventional nitride laser diode structures, where electron confinement is provided by a separate high-aluminum-content AlGaN tunnel barrier layer placed over the MQW active region. The optical confinement factor (Γ) remains: comparable to that of the conventional structure, in spite of the QW's displacement from the center of the waveguide. Room-temperature CW operation was achieved with this structure     

Critical layer thickness of strained-layer InGaAs/GaAs multiple quantum wells determined by double-crystal x-ray diffraction

Double-crystal x-ray diffraction (DCXD) is shown to reveal the onset of relaxation in strained-layer InGaAs/GaAs multiple quantum well (MQW) structures. The MQW structures contain 10 nm thick In_0.16Ga_0.84As quantum wells and 55 nm thick GaAs barrier layers. As the number of periods in the structure was increased from to 3 to 15, the x-ray rocking curves were characterized by increasing distortion of superlattice interference fringes, broadening of superlattice peaks, and reduction in peak intensity. The x-ray diffraction data are correlated with an asymmetric crosshatched surface pattern as observed under Nomarski contrast microscopy. By using DCXD and Nomarski microscopy, the onset of strain relaxation in InGaAs/GaAs MQW structures was established for samples with various GaAs barrier layer thicknesses. For MQW structures in which the thickness of the barrier layers is the same or greater than that of the strained quantum wells, the critical layer thickness can be calculated according to the Matthews and Blakeslee force-balance model with dislocation formation by the single-kink mechanism.     

Influence of free carrier plasma effect on carrier-inducedrefractive index change for quantum-well lasers

The influence of the free carrier component due to the plasma effect on carrier-induced refractive index change and its dependency on polarization for multiple-quantum-well (MQW) and bulk lasers are experimentally studied. The ratios of the component to the total index change, R_fc, are 0.6, 0.4, and 0.1 for 1.3-μm MQW, 1.3-μm bulk, and 0.8-μm MQW lasers, respectively. The TM/TE polarization ratios of the component, R_TMTE/, are 0.8 and 0.3 for 1.3-μm MQW and 0.8-μm MQW lasers. The relationship between the index change and the carrier overflow (to barrier and separate confinement heterostructure layers) for MQW lasers is also discussed. Large R_fc and R_TMTE/ for the 1.3-μm MQW laser result from the carrier overflow     

Effect of barrier thickness on the carrier distribution inasymmetric multiple-quantum-well InGaAsP lasers

Four asymmetric multiple-quantum-well (AMQW) laser structures have been grown and tested. The structures were designed to study the effect of the thickness of the barriers on the distribution of carriers amongst the quantum wells by comparing the transition cavity lengths (TCL) of mirror image AMQW lasers. The TCL method provides a quantitative measure of the degree to which the uneven carrier distribution affects the net gain of wells owing to the position of the well in the active region. We experimentally demonstrate that reducing the thickness of the barrier layers from 100 to 50 Å results in a significantly more uniform carrier distribution. The thickness of the barriers is thus shown to be an important design parameter for MQW lasers     

AlGaInP yellow-green light-emitting diodes with a tensile strain barrier cladding layer

A novel tensile strain barrier cladding (TSBC) structure is proposed which can effectively increase the potential barrier of the AlGaInP yellow-green light-emitting diodes (LED's). It was found that the electroluminescence intensity of the multi-quantum well (MQW)+TSBC AlGaInP 573-nm LED is twice as large as that of the conventional MQW AlGaInP LED emitting at the same wavelength. It was also found that the MQW+TSBC AlGaInP LED is less heat sensitive than the MQW and MQW+multiquantum barrier (MQB) AlGaInP LED's. These results indicate that the MQW+TSBC LED is useful particularly under high-temperature operation.     

AlxGa1−xN/GaN multi-quantum-well ultraviolet detector based on p-i-n heterostructures

We report on characterization of a set of AlGaN/GaN multiple-quantum-well (MQW) photodetectors. The model structure used in the calculation is the p-i-n heterojunction with 20 AlGaN/GaN MQW structures in i-region. The MQW structures have 2 nm GaN quantum well width and 15 nm Al_xGa_1−xN barrier width. The cutoff wavelength of the MQW photodetectors can be tuned by adjusting the well width and barrier height. Including the polarization field effects, on increasing Al mole fraction, the transition energy decreases, the total noise increases, and the responsivity has a red shift, and so the detectivity decreases and has a red shift.     

Hole distribution in InGaAsP 1.3-μm multiple-quantum-well laserstructures with different hole confinement energies

We have investigated the hole distribution in strained InGaAsP multiple-quantum-well (MQW) structures by direct hole transport measurements with time-resolved photoluminescence spectroscopy. The results show that the hole transport time over the MQW primarily depends on the hole confinement energy in the wells and increases sharply with the well depth. A simple thermionic emission model indicates that the heavy holes escape predominantly over the light-hole barrier edge for strain-compensated MQW structures. The results are corroborated with observed laser performance data     

Influence of Si-doping on the characteristics of InGaN-GaN multiplequantum-well blue light emitting diodes

A detailed study on the effects of Si-doping in the GaN barrier layers of InGaN-GaN multiquantum well (MQW) light-emitting diodes (LEDs) has been performed. Compared with unintentionally doped samples, X-ray diffraction results indicate that Si-doping in barrier layers can improve the crystal and interfacial qualities of the InGaN-GaN MQW LEDs. It was also found that the forward voltage is 3.5 and 4.52 V, the 20-mA luminous intensity is 36.1 and 25.1 mcd for LEDs with a Si-doped barrier and an unintentionally doped barrier, respectively. These results suggests that one can significantly improve the performance of InGaN-GaN MQW LEDs by introducing Si doping in the GaN barrier layers     

Strongly improved frequency response at high-optical input powersfrom InGaAsP compensated strain MQW electroabsorption modulators

InGaAsP MQW electroabsorption modulators with and without compensated strain were fabricated and tested. Compensated strain was employed to reduce the valence band discontinuity between the well and barrier, which decreased the heavy-hole carrier escape time. A short optical pulse coupled into the modulator was used to measure the enhancement in carrier escape time from strained compensated InGaAsP quantum wells, for the first time. As a result, the strained MQW sample demonstrated an improved frequency response when operated at high optical input powers and low fields     

Enhanced relaxation oscillation frequency and reduced nonlinear K-factor in InGaAs/InGaAsP MQW lambda /4-shifted DFB lasers

The relaxation oscillation frequency, f/sub r/, of 1.55 mu m InGaAs/InGaAsP MQW lambda /4-shifted DFB lasers was doubled by increasing the carrier injection efficiency into each quantum well, which results from an optimised bandgap energy and optimised thickness of the barrier layers. The nonlinear K-factor which determines the maximum modulation bandwidth through the damping phenomenon can be reduced by adopting a p-type modulation doped MQW structure in the active layer.<>     

Static and dynamic response of multiple-quantum-wellvoltage-controlled bistable laser diodes

We have simulated the static and dynamic characteristics of voltage-controlled multiple-quantum-well (MQW) bistable laser diodes. To investigate the time response of the saturable absorber under applied electric field, we performed pump-probe measurements with picosecond resolution. The obtained differential transmission signals indicate the reduction of the carrier escape time for the saturable absorber with increasing applied electric field. The field screening effect caused by spatial change of the carrier distribution is an important factor, as is phase space filling due to the photogenerated carriers. On the basis of the time response measurements, we have designed an MQW bistable laser by solving the modified rate equation including the recovery time response of the absorption saturation, A saturable absorption region narrower than 10 μm is suitable for obtaining a low threshold device. To achieve low switching power and high switching speed, it is important to optimize the bias conditions and the MQW structures. We can expect a turn-off time of less than 10 ps, and a repetition rate of over 5 GHz from the calculations     

Multiple quantum well PIN optoelectronic devices and a method of restoring failed device characteristics

Multiple quantum well (MQW) optical modulators have a wide range of applications in fiber-optic and remote communication systems. One of the challenges in producing reliable devices is maintaining the necessary PIN electrical characteristics while having large areas of complex MQW structures for optical processing. We report the first direct correlation between crystalline material imperfections and reverse bias behavior in MQW PIN devices. Molecular beam epitaxy grown GaAs/AlGaAs and strained InGaAs/AlGaAs MQW PIN structures are examined. Defects originating in the epitaxial material provide a conducting path along the PIN junction degrading the device performance and lowering the yield. Defectectomy, a method of eliminating the crystalline defects and restoring the device characteristics and improving the yield is described.     

Carrier collection efficiency effects and improvements ofelectroabsorption devices with different barrier structures

Self-electrooptic effect devices (SEEDs) with both thick (60 Å) and thin (35 Å) quantum-well barriers were studied experimentally. Relevant device properties including responsivity, carrier collection efficiency, switching, and optical bistability behavior are compared. SEED modulator photocurrent and reflectivity data are analyzed and shown to predict S-SEED behavior. A simple yet powerful optical technique for measuring the light utilization efficiency and the carrier collection efficiency η is described and used to compare different device mesa sizes and barrier structures. The effects of η on device performance are expounded. For thin-barrier SEEDs, η is substantially improved, approaching 100%, even at bias voltages approaching zero and for small device structures     

全固源分子束外延InAsP/InGaAsP多量子阱1.55μm激光器

利用新型全固源分子束外延技术 ,对 1 .5 5 μm波段的 In As P/ In Ga As P应变多量子阱结构的生长进行了研究。实验表明 ,较低的生长温度或较大的 / 束流比有利于提高应变多量子阱材料的结构质量 ,而生长温度对材料的光学特性有较大的影响。在此基础上生长了分别限制多量子阱激光器结构 ,制作的氧化物条形宽接触激光器实现了室温脉冲工作 ,激射波长为 1 5 63 nm,阈值电流密度为 1 .4k A/ cm2 。这是国际上首次基于全固源分子束外延的 1 .5 5 μm波段 In As P/ In Ga As P多量子阱激光器的报道     

GaAs/AlGaAs环形激光器的量子阱结构优化

为了研究量子阱结构对半导体环形激光器阈值电流的影响,从F-P腔激光器的振荡条件出发,分析了半导体环形激光器的阈值电流密度与量子阱结构参量的函数关系,并推导出最佳量子阱数的表达式。利用器件仿真软件ATLAS建立环形激光器的等效模型,仿真、分析了不同工作温度下,量子阱数、阱厚及势垒厚度对阈值电流的影响。结果表明,阈值电流随量子阱数和阱厚的增加先减小后增大,存在一组最佳值;在确定合适的量子阱数和阱厚后,相对较窄的势垒厚度有助于进一步降低阈值电流;采用GaAs/AlGaAs材料体系和器件结构,其最佳量子阱结构参量为M=3,dw=20nm及db=10nm。     

The dependence of the maximum operating temperature of longwavelength semiconductor lasers on physical and material deviceparameters

An new expression relating the theoretical maximum operating temperature, T_max, of an InGaAsP-InP-based laser to adjustable device structural and material parameters, such as the cavity length, L, facet reflectivity R, transparency current density, J_th , and the modal gain coefficient β, is presented. The validity of this relationship is demonstrated through an examination of empirical results on two sets of unstrained multiple quantum-well (MQW) laser structures with different QW widths