Cunzhu Tong

Rate Equations for 1.3-

A rate-equation model, in which three discrete quantum-dot (QD) energy levels are assumed and all possible relaxation paths and carrier transport in the GaAs barrier are considered, is presented to analyze the steady-state performance of 1.3 mum undoped and doped dots-under-a-well (DUW) and dots-in-a-well (DWELL) InAs-GaAs QD lasers. DWELL QD lasers have higher saturation value of QD level occupation probabilities and characteristic temperature (T0) than that of DUW QD lasers due to the improvement of hole confinement. The p-doped QD laser shows lower threshold current density than n-doped QD laser at the same threshold condition, and the T0 of n-doped DWELL laser is higher than that of p-doped DWELL laser at room temperature. Optimized QD layer number of DUW and DWELL QD lasers with different QD density is discussed

\mu

High temperature photoluminescence up to 100°C was demonstrated from the p-doped ten-layer InAs∕InGaAs quantum dot (QD) laser structure. 1.3μm InAs QD lasers were fabricated using pulsed anodic oxidation from this structure. High output power of 882mW and low transparency current density of 5.9A∕cm2∕QD layer were obtained. Ground state (GS) lasing could be maintained from a QD laser with short cavity length of 611μm, corresponding to the maximum modal gain of 23.1cm−1 from this laser system. GS continuous wave operation up to 100°C was also demonstrated from an InAs QD laser (50×2500μm2).

m Dots-Under-a-Well and Dots-in-a-Well Self-Assembled InAs–GaAs Quantum-Dot Lasers

We have directly imaged the formation of a GaAs quantum ring (QR) using droplet epitaxy followed by annealing in arsenic ambient. Based on the atomic force micrograph measurement and the analysis of surface energy, we determine that the formation of self-assembled GaAs QRs is due to the gallium atoms diffusion and crystallization driven by the gradient of surface energy. The phenomenon that GaAs is etched by the gallium droplets is reported and analyzed. It has been demonstrated that the epitaxy layers, such as AlAs and InGaP, can be used as the etching stop layer and hence can be used to control the shape and height of the QRs.

Low transparency current density and high temperature operation from ten-layer p-doped 1.3μm InAs∕InGaAs∕GaAs quantum dot lasers

Efficient room-temperature (RT) continuous-wave (CW) lasing operation of the 1.3 µm MBE (molecular-beam epitaxy) In(Ga)As/GaAs quantum-dot (QD) top-emitting oxide-confined vertical-cavity surface-emitting diode lasers (VCSELs) for the second-generation optical-fibre communication has been achieved. In their design, a concept of a QD inside a quantum well (QW) has been utilized. The proposed In(Ga)As/GaAs QD active region is composed of five groups of three 8 nm In0.15Ga0.85As QWs, each containing one InAs QD sheet layer. In each group located close to successive anti-node positions of the optical standing wave within the 3λ cavity, QWs are separated by 32 nm GaAs barriers. Besides, at both active-region edges, additional single InGaAs QWs are located containing single QD layers. For the 10 µm diameter QD VCSELs, the RT CW threshold current of only 6.2 mA (7.9 kA cm−2), differential efficiency of 0.11 W A−1 and the maximal output power of 0.85 mW have been recorded. The experimental characteristics are in excellent agreement with theoretical ones obtained using the optical-electrical-thermal-recombination self-consistent computer model. According to this, for the 10 µm devices, the fundamental linearly polarized LP01 mode remains the dominating one up to the current of 9.1 mA. The lowest RT CW lasing threshold below 5 mA is expected for 6 µm devices.

Investigation of the fabrication mechanism of self-assembled GaAs quantum rings grown by droplet epitaxy

A self-consistent rate equation model is presented to investigate the influence of carrier relaxation on the modulation response of 1.3 mum InAs-GaAs quantum dot lasers. In this model, the carrier dynamics in GaAs barrier, relaxation pathways, and the phonon- and Auger-assisted relaxation are considered. The dependence of 3 dB bandwidth on the relaxation time and relaxation pathway is discussed. It is shown that carrier relaxation via less energy level has better carrier confinement and higher 3 dB bandwidth. The improvement of bandwidth by tunnelling injection QD structure is investigated from the point of view of relaxation pathway. The different effects of tunnelling into ground state and excited state on the 3 dB bandwidth are analyzed. The enhanced carrier relaxation by p-type modulation doping and its effect on the bandwidth are investigated. It is found that there exists a tradeoff on the improvement of bandwidth by p-doping, which is explained as the competition between the bandwidth limitation of K -factor and relaxation dynamics. Increase in the bandwidth of QD lasers by improving both the carrier relaxation dynamics and K-factor limitation is discussed.

Room-temperature continuous-wave operation of the In(Ga)As/GaAs quantum-dot VCSELs for the 1.3 µm optical-fibre communication

We have investigated the surface morphology and optical properties of InAs/InGaAs/GaAs quantum dots grown at 450, 480 and 510 °C. While the performances of QD devices utilizing InAs/InGaAs/GaAs structures have been well-documented, there have not been many research efforts on the growth optimization of InAs/InGaAs/GaAs QDs. We found that, unlike InAs/GaAs QD structures, InAs/InGaAs/GaAs QD structures benefit from a lower QD growth temperature. Evidence from the photoluminescence (PL) spectra suggests a decreasing presence of nonradiative islands and a multi-modal size distribution following the decrease in the growth temperature. Strong room temperature PL emission at 1.32 µm with a narrow full-width at half-maximum (FWHM) of 27.8 meV was obtained from the InAs/InGaAs/GaAs QD structure grown at 450 °C, as verified by the large areal density and narrow dot size distribution. This work indicates the feasibility of obtaining good surface morphology and optical properties at a low growth temperature for InAs/InGaAs/GaAs QD structures.

Carrier Relaxation and Modulation Response of 1.3-

We report the design, growth, fabrication, and characterization of a GaAs-based resonant-cavity-enhanced (RCE) GaInNAs photodetector operating at 1.55μm. The structure of the device was designed using a transfer-matrix method (TMM). By optimizing the molecular-beam epitaxy growth conditions, six GaInNAs quantum wells were used as the absorption layers. Twenty-five (25)- and 9-pair GaAs∕AlAs-distributed Bragg reflectors were grown as the bottom and top mirrors. At 1.55μm, a quantum efficiency of 33% with a full width at half maximum of 10nm was obtained. The dark current density was 3×10−7A∕cm2 at a bias of 0V and 4.3×10−5A∕cm2 at a reverse bias of 5V. The primary time response measurement shows that the device has a rise time of less than 800ps.

\mu

Postgrowth rapid thermal annealing was used to investigate the intermixing and structural changes in p-doped and undoped InAs/In0.1Ga0.9As dots-in-a-well (DWELL) structures grown by molecular beam epitaxy. Interdiffusion of In and Ga atoms caused by thermal annealing was proven from photoluminescence (PL) measurements, where blueshifts of the energy peaks were observed. The results show that p-doped quantum dot (QD) structures are more resistant to intermixing with higher thermal energy onset, and the reason is explained as the suppressed Ga diffusion resulted from the Be dopant. Rapid quenching of the integrated PL intensity at high temperature was observed in both undoped and p-doped DWELL QDs. Good agreement was obtained by fitting the integrated PL profile using two nonradiative recombination mechanisms, resulting in two activation energies that correspond to loss of carriers to nonradiative centers.

m InAs–GaAs Quantum Dot Lasers

Metal matrix composites reinforced with ceramic particles have become the most attractive material in the research and development of new materials for thermal management applications. In this work, 40–60 vol. % TiCx-TiB2/Al composites were successfully fabricated by the method of combustion synthesis and hot press consolidation in an Al-Ti-B4C system. The effect of the TiCx-TiB2 content on the microstructure and compression properties of the composites was investigated. Moreover, the abrasive wear behavior and thermo-physics properties of the TiCx-TiB2/Al composite were studied and compared with the TiCx/Al composite. The compression properties, abrasive wear behavior and thermo-physics properties of the TiCx-TiB2/Al composite are all better than those of the TiCx/Al composite, which confirms that the TiCx-TiB2/Al composite is more appropriate for application as a heat sink.

An investigation of growth temperature on the surface morphology and optical properties of 1.3 µm InAs/InGaAs/GaAs quantum dot structures

We demonstrate experimentally the stable continuous-wave (CW) single-mode operation of Bragg reflection waveguide (BRW) lasers from 10 to 100°C. The threshold characteristics, quantum efficiency, gain, and self-heating characteristics are investigated in detail. Threefold enhancement in the optical confinement is achieved using these BRW structures for a core width two times that of their edge-emitting total internal reflection counterparts. The device shows also the high gain and characteristic temperature (~197 K) under CW operation from 10 to 100°C. The mode stability is analyzed by the calculated mode reflection spectrum, injection-current-dependent lasing spectrum, and near-field patterns.