We discuss a novel approach to the optimisation of quantum dot bilayer structures grown by molecular beam epitaxy. It has been observed that vertically stacked quantum dot structures often exhibit an increase in the average size of the islands with increasing number of quantum dot layers deposited. The understanding of the In segregation induced Stranski-Krastanow transition suggests that this is due to excess of In caused by elemental segregation. In order to limit the non uniformity of the sizes along the stacks we have grown the upper layers with different concentrations of In, showing that the optimum fraction is the one predicted by the kinetic segregation model.

We report a method for accelerated nanoscale nuclear magnetic resonance imaging by detecting several signals in parallel. Our technique relies on phase multiplexing, where the signals from different nuclear spin ensembles are encoded in the phase of an ultrasensitive magnetic detector. We demonstrate this technique by simultaneously acquiring statistically polarized spin signals from two different nuclear species (1H, 19F) and from up to six spatial locations in a nanowire test sample using a magnetic resonance force microscope. We obtain one-dimensional imaging resolution better than 5 nm, and subnanometer positional accuracy.

We report on mechanical dissipation measurements carried out on thin (∼100 nm), single-crystal silicon cantilevers with varying chemical surface termination. We find that the 1–2 nm-thick native oxide layer of silicon contributes about 85% to the friction of the mechanical resonance. We show that the mechanical friction is proportional to the thickness of the oxide layer and that it crucially depends on oxide formation conditions. We further demonstrate that chemical surface protection by nitridation, liquid-phase hydrosilylation, or gas-phase hydrosilylation can inhibit rapid oxide formation in air and results in a permanent improvement of the mechanical quality factor between three- and five-fold. This improvement extends to cryogenic temperatures. Presented recipes can be directly integrated with standard cleanroom processes and may be especially beneficial for ultrasensitive nanomechanical force- and mass sensors, including silicon cantilevers, membranes, or nanowires.

We have developed ultra-broadband Super-Luminescent Emitting Diodes (SLEDs) at 840 nm with a 3-dB bandwidth of 45-75 nm. The SLEDs show high robustness against back-reflections of up to 50% with little change in coherence length, sidelobe suppression ratio and secondary peak suppression over a wide range of back-reflections. First long-term measurements do not show any signs of device degradation. Hence, these SLEDs can be employed in OCT systems without costly broadband optical isolators.

We report the on the characterisation of 1.3μm emitting GaInNAs quantum well (QW) lasers grown by molecular beam epitaxy using a plasma nitrogen source. Through the optimization of the structural and optical properties as a function of substrate temperature and nitrogen flux conditions, we show that high optical quality structures, which exhibit good room temperature photoluminescence intensity and photoluminescence linewidths <10meV at low temperature, can be routinely achieved. To obtain 1.3μm emission, we employed a structure containing quantum wells with an indium content of 40% and a nitrogen content of 2.5% which have low nitrogen content (1%) lattice matched quaternary GaInNAs barriers, the latter enabling us to grow thick barrier structures without introducing further strain. For unmounted and uncoated 15μm ridge waveguide lasers we have achieved threshold current densities as low as 377Acm-2 for a 3 QW and record low value of 178Acm-2 for a single QW device emitting above 1310nm. The devices show excellent temperature characteristics with characteristic temperatures >90°C observed in several structures. In comparison to GaInAs quantum well lasers, the results show that at this composition (2.5%) there is no appreciable degradation of performance due to the presence of nitrogen in these samples. Increasing the nitrogen content by 1% was observed to shift the wavelength to 1390nm, but with a threshold current density increased by a factor of 2 to 830Acm-2. The results also indicate that although high quality GaInNAs lasers can be achieved at wavelengths suitable for the 1.31μm optical fibre waveband, the performance of devices with higher N content, and therefore with emission at longer wavelength, are degraded.

We have investigated the effects of growth temperature on the properties of 1.6μm GaInNAs∕GaAs multilayer quantum wells (MQWs). Strong room-temperature optical efficiency is obtained at 1.58μm for the sample grown at 375°C. However, the photoluminescence intensities with emission at similar wavelength are dramatically degraded for the samples grown at 350 and 400°C. Structural investigations show that compositional modulation and defects occurred in the sample grown at 400°C and possible point defects within the MQWs grown at 350°C. Based on these observations, the mechanism of effects of growth temperature on near-1.55-μm GaInNAs∕GaAs MQWs is discussed.

The analysis by transmission electron microscopy of GaInNAs/GaAs(001) quantum wells grown at different temperatures in the range 375–420 °C is reported. Our results with the 220BF reflection have shown the existence of periodic strain contrasts in all the wells, associated with composition fluctuations in the alloy. These contrasts are more pronounced with increasing growth temperature, revealing a kinetic limitation for the formation of the phase separation. With the theoretical equation proposed by Cahn and the amplitude of the intensity profiles taken from 220BF micrographs, the activation energy for surface diffusion in GaInNAs is calculated.

Composition modulation observed in GaInNAs quantum wells imposes an important handicap to their potential application within optical components, particularly as the indium and nitrogen contents are increased to reach longer wavelengths. In this paper, we compare our experimental results of phase separation in GaInNAs quantum wells grown at different temperatures with recent theoretical models of spinodal decomposition from the literature. This comparison has shown that the regular solution approximation, which explains the higher composition modulation compared to GaInAs samples, provides a more appropriate explanation of GaInNAs decomposition than the usual delta lattice-parameter approximation. Transmission electron microscopy shows no composition modulation contrasts with the chemical sensitive 002 dark field reflection and a strong increase in the intensity of the strain contrasts observed with 220 bright field reflection as the growth temperature increases from 360to460°C. These observations can be explained by an uncoupling between N and In composition profiles forming separate In-rich and N-rich regions according to the regular solution approximation model. We therefore believe that the compositional fluctuations in GaInNAs are not only due to GaInAs decomposition, but that an uncoupled modulation of the III and V elements is also present.

The effects of a thin GaAsSb strain-reducing layer on the optical properties of InAs∕GaAs quantum dots (QDs) are investigated. With increasing Sb composition, the room-temperature emission wavelength of the InAs QDs increases to ∼1.43μm. For Sb compositions above 14%, the system becomes Type II, with a decrease of the photoluminescence (PL) efficiency. At a composition of 14%, the room-temperature PL efficiency is maximized, and is also significantly enhanced when compared to that of conventional InGaAs-capped InAs QDs grown under the same conditions. Room-temperature ground-state lasing at 1.292μm is demonstrated for an InAs∕GaAsSb∕GaAs structure.

A study by transmission electron microscopy of the influence of the In and N contents in the ranges of 20%–35% and 1.1%–3%, respectively, on the microstructure of Ga1−xInxNyAs1−y quantum wells is presented. Frank dislocation loops characterized as extrinsic have been found in the samples with x⩾0.25. In these structures, threading dislocations appear as a consequence of the unfaulting of the loops for y⩾0.014. An analysis of the density and size of the dislocation loops has provided an estimation of the critical radius for the unfaulting process. A model for this critical radius of the unfaulting process of extrinsic Frank loops is proposed. From this model and experimental values of critical radius, an estimation of the stacking fault energy of the GaInNAs alloy has been made. We have found a reduction in the stacking fault energy of the GaInNAs alloys when increasing the N content from 1.4% to 2.3% in good agreement with the theoretical estimation of the stacking fault energies of zinc-blende GaN and InN.

The effects of inserting GaInNAs single or stepped strain-mediating layers between GaInNAs quantum wells and the surrounding matrix on the structural and optical properties of 1.3μm GaInNAs∕GaAs multiple quantum-well (QW) structures have been investigated. Compared to control samples with simple GaInNAs multiple quantum wells, samples with strain-mediating layers exhibit improved optical and structural properties. A further strong enhancement of photoluminescence efficiency at room temperature and a remarkable reduction of emission linewidth of GaInNAs multiple QWs have been observed using a compositionally stepped strain-mediating layer. These results imply that the strain profile between the quantum well and the surrounding matrix has a major effect on the optical and structural qualities of GaInNAs quantum wells.

We have studied phase separation in Ga0.62In0.38N0.023As/GaAs (001) multi-quantum wells grown at different temperatures in the range 360 degrees C-460 degrees C. Our results by Transmission Electron Microscopy have shown strain contrasts with 220BF reflection in all the wells, which are more pronounced on increasing the growth temperature. This has been attributed to enhanced phase separation in the alloy. However, contrast modulation was not observed with 002DF reflection. Both results could be explained by the existence of N rich regions and low In segregation regarding the average composition. Experimental and theoretical results point at a large effect of the introduction of N in the phase separation of GaInAsN alloys. However, these models do not explain that the magnitude of the composition modulation increases with the temperature and it is necessary to introduce kinetic factors to predict the phase separation for different growth conditions. (c) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We have studied by transmission electron microscopy the defect generation in GaInNAs quantum wells when increasing the In and N contents in the range 20–35% and 1.3–2.3%, respectively. This analysis has shown the appearance of extrinsic Frank dislocation loops for In ≥ 25%, and of threading dislocations for In=35% and N ≥ 1.4%. It is proposed that the threading dislocations are formed from the unfaulting of the Frank loops. A new theoretical model for the process of unfaulting of extrinsic loops is proposed, which has allowed us to calculate the stacking fault energy in the GaInNAs alloy.

We have analysed by Transmission Electron Microscopy and Photo luminescence (PL) the structural and optical properties of Ga1-xInxNyAs1-y quantum wells with Indium and Nitrogen contents in the range 0.20 < x < 0.35 and 0.013 < y < 0.023, respectively. Our results have shown that a degradation of the structural quality of the wells when increasing the In content takes part in two distinct steps. In the first one, an undulation of the surface of the wells on raising the In content from 20% to 25% has been observed. This is accompanied by the appearance of a high density of extrinsic dislocation loops, probably due to a high local stress concentration in the material. For these samples the PL spectra show intense and narrow emission peaks at 1.1-1.2 mu m, therefore the existence of dislocation loops seems not to affect considerably the optical efficiency of the material at this stage. In a second step, on increasing the In composition to 35%, the PL emission efficiency of the wells is severely degraded and the samples show the appearance of threading dislocations. An unfaulting of the dislocation loops as a consequence of a coalesce reaction with two Shockley partials is proposed as the mechanism for the formation of the observed threading dislocations. (c) 2005 Elsevier B.V. All rights reserved.

An analysis of the effect of different growth parameters in the optical and structural properties of InGaAsN quantum wells (QWs) on misoriented (111)B GaAs substrates is presented. The nitrogen incorporation and crystal quality were found to be dependent on the arsenic flux used. The effect of the low temperature GaAs as cap layers for the QWs was studied by atomic force microscopy and by reflection high energy electron diffraction observations of the growing surface. New features in the (2×2) reconstruction were observed whilst growing at low temperature under a nitrogen atmosphere in the chamber, possibly due to competition between As and N species on the surface. This study has led to the fabrication of QW structures emitting at wavelengths above 1.5μm, the longest reported up to date for this kind of substrates.