Mahdad Sadeghi

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.

Mode locking a 1550 nm semiconductor disk laser by using a GaInNAs saturable absorber

Passive mode locking of an optically pumped, InP-based, 1550 nm semiconductor disk laser by using a GaInNAs saturable absorber mirror is demonstrated. To reduce material heating and enable high-power operation, a 50 microm thick diamond heat spreader is bonded to the surface of the gain chip. The laser operates at a repetition frequency of 2.97 GHz and emits near-transform-limited 3.2 ps pulses with an average output power of 120 mW.

A 0.2-W Heterostructure Barrier Varactor Frequency Tripler at 113 GHz

We present a high-power InAlAs/InGaAs/InP heterostructure barrier varactor (HBV) frequency tripler. The HBV device topology was designed for efficient thermal dissipation and high efficiency. To verify simulations, the device was flip-chip soldered onto embedding microstrip circuitry on an aluminum nitride substrate. This hybrid circuit was then mounted in a waveguide block without any movable tuners. From the resulting RF measurements, the maximum output power was 195 mW at 113 GHz, with a conversion efficiency of 15%. The measured 3-dB bandwidth was 1.5%

Large ground-to-first-excited-state transition energy separation for InAs quantum dots emitting at 1.3 μm

By capping InAs quantum dots (QDs) with a thin intermediate layer of InAlAs instead of GaAs, the radiative transition wavelengths are redshifted. Surface morphology studies confirm that the redshift is due to a better preserved QD height as compared with capping by GaAs only. In contrast, the energy levels are blueshifted when using AlGaAs instead of GaAs as the barrier material. In both cases, the energy separation between the ground and the first-excited state increases significantly. Combining these approaches, we demonstrate InAs QDs with a record transition energy separation of 108 meV and ground-state emission at 1.3 μm.

Effects of Lateral Diffusion on the Temperature Sensitivity of the Threshold Current for 1.3-

We present an experimental and theoretical investigation of the temperature dependence of the threshold current for double quantum well GaInNAs-GaAs lasers in the temperature range 10 degC-110 degC. Pulsed measurements of the threshold current have been performed on broad and narrow ridge wave guide (RWG) lasers. The narrow RWG lasers exhibit high characteristic temperatures (T0) of 200 K up to a critical temperature (Tc), above which T0 is reduced by approximately a factor of 2. The T0-values for broad RWG lasers are significantly lower than those for the narrow RWG lasers, with characteristic temperatures on the order of 100 (60) K below (above) Tc. Numerical simulations, using a model that accounts for lateral diffusion effects, show good agreement with experimental data and reveal that a weakly temperature dependent lateral diffusion current dominates the threshold current for narrow RWG lasers.

\mu{\hbox {m}}

We present a study of the optimised growth conditions for InAs quantum dots (QDs) grown on GaAs substrates by solid source molecular beam epitaxy (SSMBE). Growth conditions for best luminescence intensity and linewidth were found within narrow windows of substrate temperature (500-520 C) and nominal InAs layer thickness (3.3-3.7 monolayers). The emission wavelength of such InAs QDs capped by GaAs was around 1.24 μm. However, this is red-shifted to 1.3 μm or more by capping the InAs QDs with a thin layer of In x Ga 1 x As. The results show that both In content and thickness of the capping layer can be used to tune the emission wavelength. Atomic force microscopy images show that the surface recovers to two-dimensional when depositing In 0.2 Ga 0.8 As while remaining three-dimensional when depositing In 0.4 Ga 0.6 As.

Double Quantum-Well GaInNAs–GaAs Lasers

We investigate the effects of doping and grading slope on the surface and structure of linearly alloy graded InGaAs buffers. It is found that the Be doping can improve material properties, resulting in smaller surface roughness and a lower threading dislocation density, while the Si doping has an opposite effect. The effect is strongly dependent on the grading slope. A moderate In grading slope is preferable for the strain relaxation and the minimization of the negative effect of Si doping. Physical mechanisms are proposed to explain the experimental observations. Since doping is essential for many types of optoelectronic devices, these results are valuable for improving the material properties and performance of metamorphic devices.

Optimisation of MBE growth conditions for InAs quantum dots on (001) GaAs for 1.3 μm luminescence

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.

Effects of doping and grading slope on surface and structure of metamorphic InGaAs buffers on GaAs substrates

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

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

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.