A study of impurity-free vacancy disordering in GaAs-AlGaAs forimproved modeling

A study of the parameters of the process of impurity-free vacancy disordering (IFVD) of GaAs-AlGaAs quantum-well structures is presented. The study includes photoluminescence excitation measurements which show that the as-grown barrier/well interface is better fitted by an exponential profile than a square profile. This has a significant effect on the intermixed diffusion profiles. Also, deep level transient spectroscopy measurements have been conducted on samples that were processed using IFVD. The measurements show an elevated concentration of the trap EL2 in the processed samples, which is known to be related to As antisites. The concentration of such defects agrees with the concentration calculated for IFVD to within an order of magnitude, suggesting a correlation between the point defects required for IFVD and EL2. Finally, temporally and spatially resolved photoluminescence measurements were conducted on processed samples which indicate a factor of 3 reduction in the photogenerated carrier life time after undergoing IFVD. A spatial resolution better than 3 μm has been observed     

Dependence of dielectric-cap quantum-well disordering ofGaAs-AlGaAs quantum-well structure on the hydrogen content in SiNx capping layer

Dielectric-cap quantum-well disordering of GaAs-AlGaAs multiple-quantum-well (MQW) structure was carried out using SiN_x capping layer grown by plasma enhanced chemical vapor deposition. There was a dependence of quantum-well disordering (QWD) on the hydrogen content in the SiN_x capping layer, which was varied by changing the NH₃ flow rate during the film growth. The degree of QWD increased with increasing of hydrogen content in the Si_x capping layer. The degree of QWD with SIN, capping layer grown at higher NH₃ flow rate was comparable to that with a 300-nm-thick SiO₂ capping layer at the same rapid thermal annealing condition. This result implies the possibility of obtaining spatially selective disordered MQW structure using SiN_x capping layers grown at different NH₃ flow rates. The effect of different SiN_x capping layers on QWD was characterized semiquantitatively by introducing relative vacancy density     

Threshold conditions for an ultraviolet wavelength GaN quantum-welllaser

This paper describes an analysis of the threshold conditions for a GaN-AlGaN strained quantum-well (QW) laser. Gain spectra are computed using a many-body microscopic laser theory. The spontaneous emission rates are extracted from the gain spectra using a phenomenological expression based on energy conservation arguments. From the gain and spontaneous emission spectra, threshold current densities are estimated. Inhomogeneous broadening due to spatial variations in QW thickness are included in the analysis. Gain-current characteristics are determined for a number of laser heterostructure designs where the GaN QW width and Al composition of the AlGaN barrier material are varied     

GaInNAsSb for 1.3-1.6-/spl mu/m-long wavelength lasers grown by molecular beam epitaxy

High-efficiency optical emission past 1.3 /spl mu/m of GaInNAs on GaAs, with an ultimate goal of a high-power 1.55-/spl mu/m vertical-cavity surface-emitting laser (VCSEL), has proven to be elusive. While GaInNAs could theoretically be grown lattice-matched to GaAs with a very small bandgap, wavelengths are actually limited by the N solubility limit and the high In strain limit. By adding Sb to the GaInNAs quaternary, we have observed a remarkable shift toward longer luminescent wavelengths while maintaining high intensity. The increase in strain of these new alloys necessitates the use of tensile strain compensating GaNAs barriers around quantum-well (QW) structures. With the incorporation of Sb and using In concentrations as high as 40%, high-intensity photoluminescence (PL) was observed as long as 1.6 /spl mu/m. PL at 1.5 /spl mu/m was measured with peak intensity over 50% of the best 1.3 /spl mu/m GaInNAs samples grown. Three QW GaIn-NAsSb in-plane lasers were fabricated with room-temperature pulsed operation out to 1.49 /spl mu/m.     

Room-temperature operation of GaInAsN-GaAs laser diodes in the1.5-μm range

GaInAsN-GaAs double quantum-well (DQW) laser structures emitting in the 1.5-μm range were grown by solid source molecular beam epitaxy using a radio frequency plasma source for nitrogen activation. Lasing operation in the 1.5-μm wavelength region has been realized for fabricated ridge waveguide laser diodes (LDs) under pulsed condition up to record high temperatures of 80°C resulting in an emission wavelength of 1540 nm. This is the highest emission wavelength for laser diode operation based on GaAs. In addition, to investigate the optical properties of the active region, photoluminescence studies of underlying GaInAsN-GaAs QW structures emitting at wavelengths up to 1.55 μm are presented     

Comprehensive modeling of diffused quantum-well vertical-cavitysurface-emitting lasers

A numerical model for investigating the thermal, electrical, and optical characteristics of vertical-cavity surface-emitting: lasers (VCSELs) with a diffused quantum-well (QW) structure is presented. In the model, the quasi-three-dimensional (quasi-3-D) distribution of temperature, voltage and optical fields as well as the quasi-two-dimensional (quasi-2-D) diffusion and recombination of carrier concentration inside the QW active layer are calculated in a self-consistent manner. In addition, the quasi-3-D distribution of implanted ions before and after thermal annealing are computed. The variation of electrical conductivity and absorption loss as well as the influence of impurity induced compositional disordering on the optical gain and refractive index of the QW active layer are also taken into consideration. Using this model, the steady-state characteristics of diffused QW VCSELs are studied theoretically. It is shown that significant improvement of stable single-mode operation can be obtained using diffused QW structure     

Influences of unconfined states on the optical properties ofquantum-well structures

The population of the unconfined states, with energies above the band edge of the barrier layers, can be significant in some regions of the active volume in high power lasers and amplifiers. This paper analyzes the influences of these states on optical properties, such as gain, refractive index, differential gain, and linewidth enhancement factor, for different quantum-well (QW) structures. Our results show that at high excitation levels, the unconfined band contributions to the real part of the optical susceptibility can be significant, especially in structures with weak quantum confinement potentials. This is in agreement with recent measurements of peak gain and carrier-induced refractive index change versus carrier density, for InGaAs-GaAs QW laser structures     

Quantitative model for the kinetics of compositional intermixing inGaAs-AlGaAs quantum-confined heterostructures

A quantitative atomic-scale model for the kinetics of intermixing in GaAs-AlGaAs quantum-confined heterostructures is presented. The model takes into account the statistical nature of the defect diffusion through heterostructures and calculates its effect on the Ga-Al interdiffusion across the associated interfaces. The model has been validated by successfully predicting the observed amounts of bandgap shift induced by the process of hydrogen plasma induced defect layer intermixing, as well as for the process of impurity-free vacancy disordering using SiO₂ caps. Good agreement between calculated and measured bandgap shifts has been observed. Values of the group-III vacancy diffusion coefficient, where the agreement took place, are between 2 and 3×exp[-2.72/k_BT] cm²·s^-1     

Modulation of the second-order nonlinear tensor components inmultiple-quantum-well structures

In this paper, we present experimental results which demonstrate that quantum-well intermixing techniques can be used to modulate the magnitude of the second-order nonlinear coefficient χ⁽²⁾. Impurity-free vacancy disordering with SiO₂ and Ga₂O₃ caps was used to modulate the position of the band edge and hence, the magnitude of χ_eff⁽²⁾ . Using a coupled quantum-well structure we were able to demonstrate modulation of the d₃₃ tensor components associated with the asymmetric structure and of the d₁₄ component associated with the bulk crystal structure     

Interdiffusion induced modification of surface-acoustic-waveAlGaAs-GaAs quantum-well modulators

A theoretical study of short period AlGaAs-GaAs diffused quantum-well (QW) absorption modulators operated by using surface acoustic waves (SAWs) is carried out in this paper. The as-grown QW structure is optimized and interdiffusion is used to fine tune the modulation performance. The results show that a stack of QWs can be developed at the top surface of the modulator to utilize the steep potential induced by SAWs. The optimized structure can also produce a large absorption change and thus a fast modulation speed for the same modulation depth. In comparison to previous results, the required surface acoustic wave has a longer wavelength and a lower power so that the fabrication of the interdigital transducer can be simplified. In addition, the use of interdiffusion can provide an useful fine adjustment to the operating wavelength, further enhancement of the modulation depth and an improvement in chirping with the only drawback of an increased absorption loss     

Photonic integrated circuits fabricated using ion implantation

Intermixing the wells and barriers of quantum-well (QW) laser heterostructures generally results in an increase in the bandgap energy and is accompanied by changes in the refractive index. A technique, based on ion implantation-induced QW intermixing, has been developed to enhance the quantum-well intermixing (QWI) rate in selected areas of a wafer. Such processes offer the prospect of a powerful and simple fabrication route for the integration of discrete optoelectronic devices and for forming photonic integrated circuits     

Modification of modal gain in InGaAs-GaAs quantum-well lasers dueto barrier-state carriers

This paper describes the effects of barrier-state carriers on the modal gain of InGaAs-GaAs quantum-well (QW) lasers emitting at 980 nm. Experimental studies and numerical simulations are used to examine several drive configurations, each having a unique effect on the laser response. These include compound drive current shapes, optical excitations and fast electrical drives with rise times shorter than 100 ps. We demonstrate that a large barrier-state carrier density affects the index of refraction sufficiently so as to cause a reduction in the confinement factor and modal gain which is large enough to turn the laser off     

Anodic-oxide-induced intermixing in GaAs-AlGaAs quantum-well andquantum-wire structures

Anodic oxides of GaAs were shown to enhance the intermixing in GaAs-AlGaAs quantum wells (QW) during rapid thermal processing. Proximity of the anodic oxide to the QW has been shown to influence the photoluminescence (PL) energy shift due to intermixing. Anodic oxide induced intermixing has been used to enhance quantum-wire PL in the structures grown on V-groove patterned GaAs substrates. This has been attributed to enhanced lateral confinement in these structures. Injection of defects such as group-III vacancies or interstitials was considered to be driving force for the intermixing     

Modeling and measurement of the wavelength-dependent output properties of quantum-well optical amplifiers: effects of a carrier-dependent escape time

We present a model for quantum-well (QW) semiconductor optical amplifiers (SOAs) that considers bidirectional field propagation and the carrier densities in the barrier and QW regions. Carrier capture from the barriers into the QWs and carrier escape from the QWs to the barriers are included by means of effective capture and escape times. The model incorporates the wavelength dependence of the optical response of the active region and the effects of spectral hole burning via an analytical approximation to the susceptibility of the active material, which allows one to very effectively include the wavelength dependence of the output properties of the SOA. The model is used to analyze the experimental results obtained for a multiquantum-well SOA. The simulations results show a good agreement with the experimental data when a carrier-density dependent escape time from the QW to the barrier regions is considered.     

Analytical formula of wavelength-dependent transparent current andits implications for designing wavelength sensors and WDM lasers

The wavelength dependency of transparent current is investigated theoretically and empirically for bulk and quantum-well (QW) materials. A new analytical formula is presented for obtaining accurate Fermi levels of QW structures under current injection. The transparent current can be described with an exponential function of the wavelength using two fitting parameters. The simple expression can clearly indicate the fundamental limit of the wavelength resolution when the wavelength dependency is applied for wavelength sensing. The formula was also exploited to obtain the condition of achieving uniform threshold current for wavelength division multiplexing (WDM) lasers that may cover a wide range of wavelength     

Semiconductor lasers using diffused quantum-well structures

We assess the relative merits and prospects of using diffused quantum-well (QW) structures in semiconductor lasers. First, different techniques to achieve interdiffusion are analyzed and compared. Second, recent development of semiconductor lasers using interdiffusion technique is also discussed. Third, the optical properties of diffused QWs are studied. In addition, novel design of diffused QWs structures to maintain stable single-mode operation in semiconductor lasers is proposed. Finally, brief discussion and conclusion are given     

Intimate monolithic integration of chip-scale photonic circuits

In this paper, we introduce a robust monolithic integration technique for fabricating photonic integrated circuits comprising optoelectronic devices (e.g., surface-illuminated photodetectors, waveguide quantum-well modulators, etc.) that are made of completely separate epitaxial structures and possibly reside at different locations across the wafer as necessary. Our technique is based on the combination of multiple crystal growth steps, judicious placement of epitaxial etch-stop layers, a carefully designed etch sequence, and self-planarization and passivation steps to compactly integrate optoelectronic devices. This multigrowth integration technique is broadly applicable to most III-V materials and can be exploited to fabricate sophisticated, highly integrated, multifunctional photonic integrated circuits on a single substrate. As a successful demonstration of this technique, we describe integrated photonic switches that consume only a 300 /spl times/300 /spl mu/m footprint and incorporate InGaAs photodetector mesas and InGaAsP/InP quantum-well modulator waveguides separated by 50 /spl mu/m on an InP substrate. These switches perform electrically-reconfigurable optically-controlled wavelength conversion at multi-Gb/s data rates over the entire center telecommunication wavelength band.     

Quantum-dot heterostructure lasers

Quantum-dot (QD) heterostructures are nanoscale coherent insertions of narrow-gap material in a single-crystalline matrix. These tiny structures provide unique opportunities to modify and extend all basic principles of heterostructure lasers and advance their applications. Despite early predictions, fabrication of QD heterostructure (QDHS) lasers appeared to be a much more challenging task, as compared to quantum well (QW) devices. The breakthrough occurred when techniques for self-organized growth of QD's allowed the fabrication of dense arrays of coherent islands, uniform in shape and size, and, simultaneously, free from undesirable defects. Recently, the figure of merit of QDHS lasers surpasses some of the key characteristics of QW devices in some of the most important applications     

AlGaN-GaN UV light-emitting diodes grown on SiC by metal-organicchemical vapor deposition

We report the study of the electrical and optical characteristics of AlGaN-GaN quantum-well (QW) ultraviolet light-emitting diodes grown on SiC by metal-organic chemical vapor deposition. These devices exhibit room-temperature electroluminescence emission peaked at λ = 363 nm with a narrow linewidth of Δλ = 9 nm under high-current-density dc injection. We have also applied a Mg-doped AlGaN-GaN superlattice structure as a p-cladding layer and vertical-geometry hole conduction improvement has been verified. A comparative study of the performance of light-emitting devices with single-QW and multiple-QW structures indicates that the single-QW structure is preferred     

Advances in Linear Modeling of Microwave Transistors

Heterojunction field effect transistors (HFET) based on gallium nitride (AlGaN/GaN) and metal semiconductor field effect transistors (MESFETs) based on silicon carbide (SiC) are the preferred transistors for high-power amplifier circuit designs rather than MESFETs, high electron mobility transistors (HEMTs) and pseudomorphic HEMTs based on gallium arsenide (GaAs) or indium phosphide (InP) semiconductor technology. While AlGaN/GaN and SiC are good candidates for high-power applications, GaAs and InP semiconductor technologies are the preferred transistors in low-power, low-voltage, and low-noise applications [1].