Low-bias-current direct modulation up to 33 GHz inInGaAs/GaAs/AlGaAs pseudomorphic MQW ridge-waveguide lasers

Modulation bandwidths of 24 GHz (I_bias=25 mA) and 33 GHz (I_bias=65 mA) are demonstrated for 3×100 μm² In_0.35Ga_0.65As/GaAs multiple quantum well ridge-waveguide lasers with undoped and p-doped active regions, respectively. These performance enhancements have been achieved both by lowering the growth temperature of the high-Al-mole-fraction cladding layers and by utilizing short-cavity devices, fabricated with dry-etched facets using chemically-assisted ion-beam etching. Both the undoped and p-doped lasers also demonstrate modulation current efficiency factors exceeding 5 GHz/mA^1/2, the best reported results for any semiconductor laser     

Band-edge aligned quaternary carrier barriers in InGaAs-AlGaAshigh-power diode lasers for improved high-temperature operation

A new type of band-edge aligned carrier barrier is introduced into InGaAs-AlGaAs single quantum-well (SQW) high-power diode laser structures in order to prevent thermionic emission and the overflow of carriers at elevated operating temperatures. These barriers, which are located in the direct vicinity of the active zone of the laser, are undoped to avoid free-carrier absorption. An InGaAs-AlGaAs SQW laser structure with a 10-nm-thick AlGaAsSb electron-blocking layer on the p-side of an In_0.2Ga_0.8As quantum well was realized. The composition of this layer was adjusted so that its valence-band edge matches that of the adjacent AlGaAs waveguide layer. This is to prevent any additional voltage drop or series resistance due to the injection of holes into the quantum well through the electron blocking layer. These lasers show a high characteristic temperature T ₀ of about 225 K for 1500-μm-long as-cleaved devices, which is about 60 K higher than the same laser structure without the blocking layer. Simultaneously low internal losses (α_i≈1.5 cm^-1 at 20°C) and high internal quantum efficiencies (η_i≈93% at 20°C) are achieved. No additional voltage drop or series resistance was measured. The higher temperature stability is mainly attributed to the suppression of carrier leakage and a reduced free-carrier absorption at elevated temperatures     

DC and Dynamic Characteristics of P-Doped and Tunnel Injection 1.65-μm InAs Quantum-Dash Lasers Grown on InP (001)

We have studied the characteristics of 1.65-mum InAs self-organized quantum-dash lasers grown on InP (001) substrates, wherein special techniques of p-doping of quantum dashes and tunnel injection are incorporated for the first time. We measured a very large T₀ (196 K) in p-doped quantum-dash lasers, accompanied by an increase in threshold current density (J_th~1600 A/cm² ), compared to the undoped quantum-dash lasers (T₀=76 K and J_th~950 A/cm²). The p-doped lasers exhibit a maximum 3-dB bandwidth of 8 GHz, chirp ~1.0 Aring, and alpha-parameter ~1.0 (measured at subthreshold bias conditions) at a temperature of 278 K. Similar undoped quantum-dash lasers exhibit a 3-dB bandwidth of 6 GHz. A self-consistent model, that includes Auger recombination in quantum dashes, is developed to calculate the threshold current at various temperatures. A comparison of the calculated threshold current and T₀ with measured values reveals that Auger recombination in quantum dashes plays a major role in determining the values of threshold current and T₀ in both undoped and p-doped quantum-dash lasers. While p-doping increases the gain and differential gain, the presence of wetting layer states, the relatively large inhomogeneous broadening of quantum dashes, and the substantially increased Auger recombination upon p-doping severely limit the potential benefits. Superior characteristics, including large modulation bandwidth (f_{-3 dB}~12 GHz), near-zero alpha-parameter, and very low chirp (~0.3 Aring), are achieved when the technique of tunnel injection is also utilized     

1.3-μm CW lasing characteristics of self-assembled InGaAs-GaAsquantum dots

This paper presents the lasing properties and their temperature dependence for 1.3-μm semiconductor lasers involving self-assembled InGaAs-GaAs quantum dots as the active region. High-density 1.3-μm emission dots were successfully grown by the combination of low-rate growth and InGaAs-layer overgrowth using molecular beam epitaxy. 1.3-μm ground-level CW lasing occurring at a low threshold current of 5.4 mA at 25°C with a realistic cavity length of 300 μm and high-reflectivity coatings on both facets. The internal loss of the lasers was evaluated to be about 1.2 cm^-1 from the inclination of the plots between the external quantum efficiency and the cavity length. The ground-level modal gain per dot layer was evaluated to be 1.0 cm^-1, which closely agreed with the calculation taking into account the dot density, inhomogeneous broadening, and homogeneous broadening. The characteristic temperature of threshold currents T₀ was found to depend on cavity length and the number of dot layers in the active region of the lasers. A T₀ of 82 K was obtained near room temperature, and spontaneous emission intensity as a function of injection current indicated that the nonradiative channel degraded the temperature characteristics. A low-temperature study suggested that an infinite T₀ with a low threshold current (~1 mA) is available if the nonradiative recombination process is eliminated. The investigation in this paper asserted that the improvement in surface density and radiative efficiency of quantum dots is a key to the evolution of 1.3-μm quantum-dot lasers     

Coupled drift-diffusion/quantum transmitting boundary methodsimulations of thin oxide devices with specific application to a siliconbased tunnel switch diode

We present a method of coupling drift-diffusion simulations with quantum transmitting boundary method (QTBM) tunnel current calculations. This allows self-consistent simulation of thin oxide devices in which large tunnel currents can flow. Simulated results are presented for a thin oxide Al/SiO₂/Si structure and an Al/SiO₂/n-Si/p-Si tunnel switching diode. We demonstrate the careful use of the recombination lifetime as an adjustable or relaxable parameter in order to obtain converging solutions     

Self-aligned GaAs p-channel enhancement mode MOS heterostructurefield-effect transistor

Self-aligned GaAs enhancement mode MOS heterostructure field-effect transistors (MOS-HFET) have been successfully fabricated for the first time. The MOS devices employ a Ga₂O₃ gate oxide, an undoped Al_0.75Ga_0.25As spacer layer, and undoped In_0.2Ga_0.8As as channel layer. The p-channel devices with a gate length of 0.6 μm exhibit a maximum DC transconductance g_m of 51 mS/mm which is an improvement of more than two orders of magnitude over previously reported results. With the demonstration of a complete process flow and 66% of theoretical performance, GaAs MOS technology has moved into the realm of reality     

High-Performance Quantum Dot Lasers and Integrated Optoelectronics on Si

This paper provides a review of the recent developments of self-organized In(Ga)As/Ga(Al)As quantum dot lasers grown directly on Si, as well as their on-chip integration with Si waveguides and quantum-well electroabsorption modulators. A novel dislocation reduction technique, with the incorporation of self-organized In(Ga,Al)As quantum dots as highly effective three-dimensional dislocation filters, has been developed to overcome issues associated with the material incompatibility between III-V materials and Si. With the use of this technique, quantum dot lasers grown directly on Si exhibit relatively low threshold current (J _th=900 A/cm²) and very high temperature stability (T ₀=278 K). Integrated quantum dot lasers and quantum-well electroabsorption modulators on Si have been achieved, with a coupling coefficient of more than 20% and a modulation depth of ~100% at a reverse bias of 5 V. The monolithic integration of quantum dot lasers with both amorphous and crystalline Si waveguides, fabricated using plasma-enhanced chemical-vapor deposition and membrane transfer, respectively, has also been demonstrated.     

Strain-compensated InGa(As)P-InAsP active regions for 1.3-μmwavelength lasers

The demonstration of an optimized strain compensated multiple-quantum-well (MQW) active region for use in 1.3-μm wavelength lasers is described. Utilizing narrow bandgap tensile-strained InGaAsP instead of wide bandgap InGaP barriers in strain-compensated lasers, we observe a reduction in threshold current density (Jth) from 675 to 310 A/cm² and in T₀ from 75 K to 65 K for 2-mm long seven quantum-well devices. Additionally, the lowest reported Jth for MBE grown 1.3-μm wavelength lasers of 120 A/cm² for single-quantum-well (SQW) 45-mm-long lasers was attained     

The influence of quantum-well composition on the performance ofquantum dot lasers using InAs-InGaAs dots-in-a-well (DWELL) structures

The optical performance of quantum dot lasers with different dots-in-a-well (DWELL) structures is studied as a function of the well number and the indium composition in the InGaAs quantum well (QW) surrounding the dots. While keeping the InAs quantum dot density nearly constant, the internal quantum efficiency η_i, modal gain, and characteristic temperature of 1-DWELL and 3-DWELL lasers with QW indium compositions from 10 to 20% are analyzed. Comparisons between the DWELL lasers and a conventional In_0.15Ga_0.85As strained QW laser are also made. A threshold current density as low as 16 A/cm² is achieved in a 1-DWELL laser, whereas the QW device has a threshold 7.5 times larger. It is found that η_i and the modal gain of the DWELL structure are significantly influenced by the quantum-well depth and the number of DWELL layers. The characteristic temperature T₀ and the maximum modal gain of the ground-state of the DWELL structure are found to improve with increasing indium in the QW It is inferred from the results that the QW around the dots is necessary to improve the DWELL laser's η_i for the dot densities studied     

Interband-resonant light modulation by intersubband-resonant lightin undoped quantum wells

Interband-resonant light modulation by intersubband-resonant light in undoped quantum wells is investigated. Theoretical calculation for the modulation is carried out by considering not only the excitonic interband-transition but also the continuous level transition between the conduction and valence bands. The modulation characteristics are compared with those of the modulation using n-doped quantum wells. The possibility of the modulation using undoped quantum well is successfully shown by real-time single-shot experiment using Ti-Al₂O₃ and CO₂ lasers for interband- and intersubband-resonant lights at room temperature     

A selective epitaxial SiGe/Si planar photodetector for Si-basedOEIC's

A P-i-N SiGe/Si superlattice photodetector with a planar structure has been developed for Si-based opto-electronic integrated circuits. To make the planar structure, a novel SiGe/Si selective epitaxial growth technology which uses cold wall ultrahigh-vacuum/chemical vapor deposition has been newly developed. The P-i-N planar SiGe/Si photodetector has an undoped 30-Å Si_0.9Ge_0.1/320-Å Si, 30 periods, superlattice absorption layer, a 0.1-μm P-Si buffer layer, and a 0.2-μm P⁺-Si contact layer on a bonded silicon-on-insulator (η_ext). The bonded SOI is used to increase the external quantum efficiency (η_ext) of the photodetector. Moreover, a 63-μm deep/128-μm wide trench, to achieve simple and stable coupling of an optical fiber to the photodetector, is formed in the silicon chip. The P-i-N planar photodetector exhibits a high η_ext of 25-29% with a low dark current of 0.5 pA/μm² and a high-frequency photo response of 10.5 GHz at λ=0.98 μm     

Quantum coupling effects on charging dynamics of nanocrystalline memory devices

A model is proposed to account for the impacts of the quantum coupling between the longitudinal and transverse components of the channel electron motion on the charging dynamics of memory devices. The calculations demonstrate that the quantum coupling effects on the charging dynamics of Ge NC (germanium nanocrystalline) memory devices cannot be neglected for high temperature and drift velocity of the channel electrons higher than the thermal velocity. The calculations also show that the charging current of Ge NC memory devices strongly depends on the temperature, drift velocity and effective electron mass of the tunneling oxide layer. The reduction in the barrier height caused by the quantum coupling is its origin. The sensitivity of the effective electron mass of the tunneling oxide layer on the charging current of Ge NC memory devices is a potential method to improve the performance of device.     

Structural and electrooptical characteristics of quantum dotsemitting at 1.3 μm on gallium arsenide

We present a comprehensive study of the structural and emission properties of self-assembled InAs quantum dots emitting at 1.3 μm. The dots are grown by molecular beam epitaxy on gallium arsenide substrates. Room-temperature emission at 1.3 μm is obtained by embedding the dots in an InGaAs layer. Depending on the growth structure, dot densities of 1-6×10¹⁰ cm^-2 are obtained. High dot densities are associated with large inhomogeneous broadenings, while narrow photoluminescence (PL) linewidths are obtained in low-density samples. From time-resolved PL experiments, a long carrier lifetime of ≈1.8 ns is measured at room temperature, which confirms the excellent structural quality. A fast PL rise (τ_rise=10±2 ps) is observed at all temperatures, indicating the potential for high-speed modulation. High-efficiency light-emitting diodes (LEDs) based on these dots are demonstrated, with external quantum efficiency of 1% at room temperature. This corresponds to an estimated 13% radiative efficiency. Electroluminescence spectra under high injection allow us to determine the transition energies of excited states in the dots and bidimensional states in the adjacent InGaAs quantum well     

InP/In0.53Ga0.47As heterojunction bipolartransistors with a carbon-doped base grown by MOCVD

InP/In_0.53Ga_0.47As heterojunction bipolar transistors (HBTs) utilizing a carbon-doped base have been demonstrated. The devices were grown by low-pressure metalorganic chemical vapor deposition (LP-MOCVD) using carbon tetrachloride (CCl₄) as the p-type dopant source. These devices exhibit a DC common-emitter current gain of 50 and an emitter-base junction ideality factor of 1.29 in a structure for which no undoped spacer layer was employed at the emitter-base junction. These preliminary results suggest that C-doping of In_0.53Ga_0.47As may be a suitable alternative to Zn in MOCVD-grown InP/In_0.53Ga_0.47As HBTs     

Device characteristics of (Al,Ga)As lasers with Ga(As,Sb) active layers

Device characteristics of double heterostructure lasers with Al0.4Ga0.6As confinement layers and GaAs0.99Sb0.01active layers are presented. Average emission wavelengths have been increased from 0.87 μm for undoped active layers to 0.88 μm for2 times 10^{17}cm^-3Ge doped active layers with a "wash" melt preceding the growth of the active layer and to 0.89 μ for1 times 10^{18}cm^-3Mg doped active layers grown following a "wash" melt. Threshold currents for lasers from 21 wafers are examined for several growth conditions and compared with Al0.08Ga0.92As active layer devices. Device resistance, external quantum efficiency, device degradation, and pulsed and CW threshold currents as a function of temperature are also discussed.     

An 0.18-μm CMOS for mixed digital and analog applications withzero-volt-Vth epitaxial-channel MOSFETs

An 0.18-μm CMOS technology with multi-V_ths for mixed high-speed digital and RF-analog applications has been developed. The V _ths of MOSFETs for digital circuits are 0.4 V for NMOS and -0.4 V for PMOS, respectively. In addition, there are n-MOSFET's with zero-volt-V_th for RF analog circuits. The zero-volt-V_th MOSFETs were made by using undoped epitaxial layer for the channel regions. Though the epitaxial film was grown by reduced pressure chemical vapor deposition (RP-CVD) at 750°C, the film quality is as good as the bulk silicon because high pre-heating temperature (940°C for 30 s) is used in H₂ atmosphere before the epitaxial growth. The epitaxial channel MOSFET shows higher peak g_m and f_T values than those of bulk cases. Furthermore, the g_m and f_T values of the epitaxial channel MOSFET show significantly improved performances under the lower supply voltage compared with those of bulk. This is very important for RF analog application for low supply voltage. The undoped-epitaxial-channel MOSFETs with zero-V_th will become a key to realize high-performance and low-power CMOS devices for mixed digital and RF-analog applications     

High-performance quantum cascade lasers with electric-field-freeundoped superlattice

An optimized design of quantum cascade lasers with electric field free undoped superlattice active regions is presented. In these structures the superlattice is engineered so that: (1) the first two extended states of the upper miniband are separated by an optical phonon to avoid phonon bottleneck effects and concentrate the injected electron density in the lower state and (2) the oscillator strength of the laser transition is maximized. The injectors' doping profile is also optimized by concentrating the doping in a single quantum well to reduce the electron density in the active material. These design changes result in major improvements of the pulse/continuous-wave performance such as a weak temperature dependence of threshold (T₀=167 K), high peak powers (100-200 mW at 300 K) and higher CW operating temperatures for devices emitting around at λ~8.5 μm     

High-finesse resonant-cavity photodetectors with an adjustableresonance frequency

High speeds, high external quantum efficiencies, narrow spectral linewidths, and convenience in coupling make resonant-cavity photodetectors (RECAP's) good candidates for telecommunication applications. In this paper, we present analytical expressions for the design of RECAP's with narrow spectral linewidths and high quantum efficiencies. We also present experimental results on a RECAP having an operating wavelength λ₀≈1.3 μm with a very narrow spectral response. The absorption takes place in a thin In_0.53Ga_0.47As layer placed in an InP cavity. The InP p-i-n structure was wafer bonded to a high-reflectivity GaAs/AlAs quarter-wavelength Bragg reflector. The top mirror consisted of three pairs of a ZnSe/CaF₂ quarter-wavelength stack (QWS). A spectral linewidth of 1.8 nm was obtained with an external quantum efficiency of 48%. We also demonstrate that the spectral response can be tailored by etching the top layer of the microcavity. The results are found to agree well with those obtained from analytical expressions derived on the assumption of linear-phase Bragg reflectors as well as detailed simulations performed using the transfer matrix method     

发光层厚度对基于CdSSe/ZnS量子点LED性能的影响

采用粒径约为10 nm的CdSSe/ZnS量子点层作为发光层,制备了叠层结构的量子点发光器件,研究了量子点层厚度对其薄膜形貌及量子点发光二极管性能的影响.原子力显微镜测试结果表明:量子点层过厚时,量子点颗粒发生团聚,且随着厚度的降低,团聚现象减弱;当量子点层厚度和量子点粒径相当时(约为10 nm),量子点呈单层排列且团聚现象基本消失;而量子点层厚度低于10 nm时,薄膜出现孔洞缺陷.器件的电流-电压-亮度等测试结果表明:量子点发光二极管中量子点层厚度与器件的光电特性密切相关,量子点层厚度为10 nm的器件光电性能最优,具有最低的启亮电压4.2V,最高的亮度446 cd/m2及最高的电流效率0.2 cd/A.这种通过控制旋涂转速改变量子点层厚度的方法操作简单、重复性好,对QD-LED的研究具有一定应用价值.     

Electro‐Optics of Colloidal Quantum Dot Solids for Thin‐Film Solar Cells

The electro‐optics of thin‐film stacks within photovoltaic devices plays a critical role for the exciton and charge generation and therefore the photovoltaic performance. The complex refractive indexes of each layer in heterojunction colloidal quantum dot (CQD) solar cells are measured and the optical electric field is simulated using the transfer matrix formalism. The exciton generation rate and the photocurrent density as a function of the quantum dot solid thickness are calculated and the results from the simulations are found to agree well with the experimentally determined results. It can therefore be concluded that a quantum dot solid may be modeled with this approach, which is of general interest for this type of materials. Optimization of the CQD solar cell is performed by using the optical simulations and a maximum solar energy conversion efficiency of 6.5% is reached for a CQD solid thickness of 300 nm.