Thermal Effects and Small Signal Modulation of 1.3-µm InAs/GaAs Self-Assembled Quantum-Dot Lasers.

We investigate the influence of thermal effects on the high-speed performance of 1.3-µm InAs/GaAs quantum-dot lasers in a wide temperature range (5-50°C). Ridge waveguide devices with 1.1 mm cavity length exhibit small signal modulation bandwidths of 7.51 GHz at 5°C and 3.98 GHz at 50°C. Temperature-dependent K-factor, differential gain, and gain compression factor are studied. While the intrinsic damping-limited modulation bandwidth is as high as 23 GHz, the actual modulation bandwidth is limited by carrier thermalization under continuous wave operation. Saturation of the resonance frequency was found to be the result of thermal reduction in the differential gain, which may originate from carrier thermalization.

High responsivity GaNAsSb p-i-n photodetectors at 1.3 microm grown by radio-frequency nitrogen plasma-assisted molecular beam epitaxy.

GaNAsSb/GaAs p-i-n photo notdetectors with an intrinsic GaNAsSb photoabsorption layer grown at 350 degrees C, 400 degrees C, 440 degrees C and 480 degrees C, have been prepared using radio-frequency nitrogen plasma-assisted molecular beam epitaxy in conjunction with a valved antimony cracker source. The i-GaNAsSb photoabsorption layer contains 3.3% of nitrogen and 8% of antimony, resulting in DC photo-response up to wavelengths of 1350 nm. The device with i-GaNAsSb layer grown at 350 degrees C exhibits extremely high photoresponsivity of 12A/W at 1.3 microm. These photodetectors show characteristics which strongly suggest the presence of carrier avalanche process at reverse bias less than 5V.

Temperature and excitation density dependent photoluminescence of sputtering-induced GaAs/AlGaAs quantum dots.

GaAs/AlGaAs quantum dots (QDs) are fabricated by low-energy ion beam sputtering and molecular beam epitaxy (MBE) re-growth. Temperature (6.5-78 K) and excitation power density (0.49-3.06 W cm(-2)) dependent photoluminescence (PL) are presented and discussed in detail. The low-temperature PL emission at 720 nm is attributed to GaAs QDs with height of ∼6.1 nm and base width of ∼23 nm, calculated based on the quantum box model with infinite potential barrier. The calculated QD dimensions are in good agreement with those obtained from atomic force microscopy (AFM) analysis. Nonradiative recombination and Auger-assisted recombination are found to be the main PL quenching mechanisms at high temperature.

The effect of nitrogen pressure during molecular beam epitaxy growth of InAsN quantum dots.

The influence of N flux during molecular beam epitaxy growth of InAsN quantum dots was studied. Growth of InAsN dots under high N flux was shown to give rise to an abnormal growth behaviour compared to InAs dots and InAsN dots with lower nitrogen content. Cubic In(x)Ga(1-x)N (x = 0.21 ± 0.01) crystallites were found in samples grown with an excessive N flux. The crystallites are likely to form ∼0.6 monolayers (MLs) after the quantum dots have nucleated, when the quantum dot changes growth mode. In addition, it is shown that a bimodal size distribution of InAsN quantum dots was generated in the wetting layer during the dot growth, as opposed to nucleation at N-induced dislocations at the substrate surface. The bimodal distribution may be explained by an increased energy barrier, in the presence of nitrogen, for atomic incorporation into the dots.

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

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 atom's 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.

The influence of substrate temperature on InAsN quantum dots grown by molecular beam epitaxy.

The effect of substrate temperature, 390-480 °C, during molecular beam epitaxy growth of InAsN quantum dots has been studied. The quantum dot formation was studied in situ, and it is shown that the quantum dots are close to fully relaxed within 4 monolayers (ML) of InAsN deposition. Further, the indium concentration was estimated to be 84%, 67%, 55% and 31% for 4 ML thick quantum dots grown at 390, 420, 450 and 480 °C, respectively. Thus, Ga incorporation was demonstrated at all substrate temperatures. The dot diameter and height increased from 23 to 38 nm, and 2.5 to 8.9 nm, respectively, when the growth temperature was increased from 390 to 480 °C. The 5 K photoluminescence intensity and wavelength both increased with substrate temperature.

Investigation of Semiconductor Quantum Dots for Waveguide Electroabsorption Modulator.

In this work, we investigated the use of 10-layer InAs quantum dot (QD) as active region of an electroabsorption modulator (EAM). The QD-EAM is a p-i-n ridge waveguide structure with intrinsic layer thickness of 0.4 mum, width of 10 mum, and length of 1.0 mm. Photocurrent measurement reveals a Stark shift of ~5 meV (~7 nm) at reverse bias of 3 V (75 kV/cm) and broadening of the resonance peak due to field ionization of electrons and holes was observed for E-field larger than 25 kV/cm. Investigation at wavelength range of 1,300-1320 nm reveals that the largest absorption change occurs at 1317 nm. Optical transmission measurement at this wavelength shows insertion loss of ~8 dB, and extinction ratio of ~5 dB at reverse bias of 5 V. Consequently, methods to improve the performance of the QD-EAM are proposed. We believe that QDs are promising for EAM and the performance of QD-EAM will improve with increasing research efforts.

Comparative analysis of cavity length-dependent temperature sensitivity of GaInNAs quantum dot lasers and quantum well lasers.

Self-assembled GaInNAs/GaAsN single-layer quantum dot (QD) lasers, grown using solid source molecular beam epitaxy, have been fabricated and characterized. A high output power of 40.76 mW/facet was obtained from a GaInNAs QD laser with dimensions of 50 × 700 µm(2) at 10 °C. Temperature-dependent measurements were carried out on the GaInNAs QD lasers of different cavity lengths. For comparison, temperature-dependent measurements were also performed on GaInNAs single quantum well (SQW) and triple QW (TQW) lasers. Unlike the relationship between cavity length and T(0) in GaInNAs SQW/TQW lasers, longer-cavity GaInNAs QD lasers (50 × 1700 µm(2)) showed a lower T(0) of 65.1 K, which is believed to be due to non-uniformity of the GaInNAs QD layer. Furthermore, compared to GaInNAs SQW lasers, a significant improvement in temperature sensitivity was observed in the TQW GaInNAs lasers. This is attributed to a reduction in the relative contribution of the Auger recombination current and suppression of heavy-hole leakage in the TQW laser structures.

A novel multi-walled carbon nanotube-based biosensor for glucose detection.

The bioelectrochemical characteristics of a novel multi-walled carbon nanotube (MWNT)-based biosensor for glucose detection are studied and compared with those of glassy carbon (GC)-based biosensor. The MWNT-based biosensor exhibits a strong glucose response at applied potentials of 0.65 and 0.45 V versus Ag/AgCl, respectively, while GC-based biosensor shows a weak glucose response at 0.65 V and no response at 0.45 V. Besides, the MWNT-based biosensor shows a high stability of 86.7% of the initial activity to glucose after four-month storage, much higher than 37.2%, the corresponding value for a GC-based biosensor. The detection mechanism of the MWNT-based biosensor is also discussed in detail.

Thermoluminescence in CVD diamond films: application to actinometric dosimetry.

Diamond is considered a tissue-equivalent material since its atomic number (Z =6) is close to the effective atomic number of biological tissue (Z =7.42). Such a situation makes it suitable for radiation detection purposes in medical applications. In the present work the analysis is reported of the thermoluminescence (TL) and dosimetric features of chemically vapour deposited (CVD) diamond film samples subjected to ultraviolet (UV) irradiation in the actinometric region. The TL glow curve shows peaks at 120, 220), 320 and 370 degrees C. The 120 and 370 degrees C peaks are too weak and the first one fades away in a few seconds after exposure. The overall room temperature fading shows a 50% TL decay 30 min after exposure. The 320 degrees C glow peak is considered to be the most adequate for dosimetric applications due to its low fading and linear TL behaviour as a function of UV dose in the 180-260 nm range. The TL excitation spectrum presents a broad band with at least two overlapped components around 205 and 220 nm. The results indicate that the TL behaviour of CVD diamond film can be a good alternative to the currently available dosemeter and detector in the actinometric region as well as in clinical and medical applications.

Surface acoustic wave reflection from diamond-like carbon thin film reflecting arrays on LiNbO3 substrates.

Surface acoustic wave (SAW) reflection from diamond-like carbon (DLC) strip reflecting arrays on Y-Z LiNbO3 is investigated. The reflection from DLC strips with triangular cross section has been observed. Reflection increases in alternating DLC and Al strips in 90 degrees reflecting arrays in comparison with pure Al structures. The values of reflection coefficient per period in the slanted reflecting arrays are estimated to be about 1.0% for pure DLC strips (height to wavelength ratio equal to 0.02), 2% for uniform Al coating on DLC reflecting arrays, and 3.5% for alternating DLC and Al strips. This value is higher than that for pure Al strips by about 0.7%. Reflection properties are briefly discussed, and preparation technique is presented.