Yong Qiang Wei

Influence of a thin GaAs cap layer on structural and optical properties of InAs quantum dots

In this letter we investigate the changes in the surface morphology and emission wavelength of InAs quantum dots (QDs) during initial GaAs encapsulation by atomic force microscopy and photoluminescence. The density (2.9×1010 cm−2) and height (7.9±0.4 nm) of the uncapped QDs decrease and saturate at 0.6×1010 cm−2 and 4 nm, respectively, after the deposition of 4 monolayers (MLs) of GaAs. A model for the evolution of surface morphology is proposed. Photoluminescence spectra of the surface dots show a wavelength shift from 1.58 to 1.22 μm when the GaAs capping layer thickness increases from 0 to 8 MLs.

Uncooled 2.5 Gb/s operation of 1.3 μm GaInNAs DQW lasers over a wide temperature range

Ridge waveguide 1.3 mum GaInNAs lasers were fabricated from high quality double quantum well material grown by molecular beam epitaxy. Short cavity (250 mum) lasers have low threshold currents and small temperature dependencies of threshold current and slope efficiency, with a characteristic temperature of the threshold current as high as 200 K. The temperature stability allows for uncooled 2.5 Gb/s operation up to temperatures as high as 110 degrees C with a constant modulation voltage and only the bias current adjusted for constant average output power. Under these conditions, an extinction ratio larger than 6 dB and a spectral rms-width smaller than 2 nm are obtained.

Dynamics and Temperature-Dependence of 1.3-

We have measured the small-signal modulation response of 1.3-mum ridge waveguide GaInNAs double quantum-well lasers over a wide range of temperatures (25 degC-110 degC) and analyzed the temperature dependence of the modulation bandwidth and the various bandwidth limiting effects. The lasers have low threshold currents and high differential efficiencies with small temperature dependencies. A short-cavity (350 mum) laser has a modulation bandwidth as high as 17 GHz at room temperature, reducing to 4 GHz at 110 degC, while a laser with a longer cavity (580 mum) maintains a bandwidth of 8.6 GHz at 110 degC. We find that at all ambient temperatures the maximum bandwidth is limited by thermal effects as the temperature increases with current due to self-heating. The reduction and subsequent saturation of the resonance frequency with increasing current is due to a reduction of the differential gain and an increase of the threshold current with increasing temperature. We find large values for the differential gain and the gain compression factor. The differential gain decreases linearly with temperature while there is only a weak temperature dependence of the gain compression. At the highest temperature we also find evidence for transport effects that increase the damping rate and reduce the intrinsic bandwidth

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We have studied the modulation bandwidth of high-speed GaInNAs double-quantum-well lasers emitting at 1.28–1.30μm. A 400μm long ridge waveguide laser exhibits a small signal modulation bandwidth of 14 GHz. The intrinsic damping limited modulation bandwidth is as high as 25 GHz (K=0.35ns), and the actual modulation bandwidth is limited by thermal effects under continuous operation. The saturation of the resonance frequency at 10 GHz was found to be the result of a thermal reduction of the differential gain and a rapid increase of the threshold current when the temperature exceeds 80 °C.

GaInNAs Double Quantum-Well Lasers

Capping of InAs quantum dots (QDs) with AlAs or GaAs causes a significant change in the structural properties of the QDs. However, there is a basic difference between these two capping materials. The GaAs capping causes a dramatic reduction of the dot density and height. AlAs capping, on the other hand, results in a partly suppressed height reduction and a higher dot density.

High-frequency modulation and bandwidth limitations of GaInNAs double-quantum-well lasers

The threshold and gain characteristics of GaInNAs single quantum well (QW) lasers with GaNAs and GaAs barriers, both emitting at 1300 nm, have been compared. The threshold current density for the laser with GaAs barriers is twice as high, presumably because of a higher monomolecular recombination rate caused by the higher N concentration in the QW. A significant difference in the spectral gain characteristics was also observed. Calculations show that this is due to a modification of the confinement potential for the conduction band electrons when incorporating N in the barriers and reducing the N concentration in the QW. An additional inhomogeneous broadening also had to be included in the calculations to obtain quantitative agreement between measured and calculated gain spectra.

Influence of initial GaAs and AlAs cap layers on InAs quantum dots grown by molecular beam epitaxy

The spectral gain characteristics of dilute-nitride zinc blende Inx Ga1-x Ny As1-y quantum wells embedded in Ga N y1 As1- y1 barriers have been investigated experimentally and theoretically. Two samples, both with the gain peak at 1300 nm, were studied for comparison. One has a high nitrogen concentration in the quantum well with the surrounding barriers being pure GaAs. The other has a lower and uniform nitrogen concentration in the quantum well and the barriers (GaNAs barriers). Measurements show the redshift of the gain peak induced by the incorporation of nitrogen and difference in the spectral gain characteristics. The energy band structures and spectral gain characteristics are analyzed theoretically using the standard eight-band kp theory. It is shown that the introduction of nitrogen atoms in the GaAs barriers reduces the barrier height for the central quantum well so that the energy sublevels in the conduction band becomes condensed. The condensation of the conduction-band energy sublevels reduces the peak gain and makes the gain spectrum narrower, in agreement with measurements.

Direct Comparison of Threshold and Gain Characteristics of 1300 nm GaInNAs lasers with GaNAs and GaAs Barriers

We report results from investigation of the optical properties of GaNAs/GaAs quantum well structures. The structures were grown by molecular-beam epitaxy at different temperatures, and subsequently postgrowth thermal treatments at different temperature were performed. The results show that the carrier localization is smaller in a structure grown at a temperature of 580 °C in comparison with a structure grown at 450 °C. Both structures also show a broaden deep level emission band. Furthermore, the deep level emission band and the carrier localization effect can be removed by thermal annealing at 650 °C in the structure grown at 450 °C. The structure quality and radiative recombination efficiency are significantly improved after annealing. However, annealing under the same condition has a negligible effect on the structure grown at 580 °C.

Energy band structure and spectral gain characteristics of dilute-nitride zinc blende InGaNAs quantum wells embedded in GaAs and GaNAs barriers

We present epitaxial growth of GaInNAs on GaAs by molecular beam epitaxy (MBE) using analog, digital and N irradiation methods. It is possible to realize GaInNAs quantum wells (QWs) with a maximum substitutional N concentration up to 6% and a strong light emission up to 1.71μm at 300K. High quality 1.3μm GaInNAs multiple QW edge emitting laser diodes have been demonstrated. The threshold current density (for a cavity of 100x1000μm2) is 300, 300, 400 and 940A/cm2 for single, double, triple and quadruple QW lasers, respectively. The maximum 3dB bandwidth reaches 17GHz and high-speed transmission at 10Gb/s up to 110°C under a constant voltage has been demonstrated.

Effect of growth temperature and post-growth thermal annealing on carrier localization and deep level emissions in GaNAs∕GaAs quantum well structures

Dilute-nitride lasers with record performances have been used to build uncooled transceivers and failure free 10 Gb/s optical transmission was achieved over 815 m of multimode Corning InfiniCor fiber under the LRM standard.