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The effect of post-growth annealing (PGA) on dot-in-well (DWELL) structures grown on Si substrates has been studied. The photoluminescence (PL) measurements showed that, compared to the DWELL structures directly grown on GaAs, the PGA process induces a distinct difference in the tuning of the emission properties. Then, transmission electron microscopy imaging of the samples revealed that PGA improved the uniformity of quantum dots (QDs) while the size of the QDs increased, in agreement with a corresponding red shift and a decrease of the full width at half maximum in the PL emission spectrum. Finally, energy-dispersive x-ray linescan provided a quantitative analysis of the composition change of DWELL grown on Si in the as-grown, 700 °C annealed, and 800 °C annealed samples. The change in the InL/GaK concentration ratio became gradual between the QDs and surrounding materials after 800 °C annealing. The analysis of the optical properties, morphology evolution, and compositional change of the QDs as a function of the annealing temperature showed good agreement.

In this work, we give a direct interpretation of micrographs of the 60° and 90° defect core at the GaAs/Si interface using aberration corrected scanning transmission electron microscopy. We investigate the post-growth annealing effects on dislocation rearrangement at the interface as well as the threading dislocations in buffer layers; finally, the density of threading dislocations has been calculated as a function of annealing temperature.

The growth temperature of spacer layers (SPLs) is investigated as a means to obtain identical layers for multilayer quantum dot (QD) structures. A 5-layer 1.3-μm InAs∕GaAs QD structure with 50-nm GaAs SPLs served as a model system. It is found that the growth temperature of the GaAs SPLs has pronounced effects on both the structural and optical properties of the InAs QDs. For GaAs SPLs grown at a low temperature of 510°C, dislocations are observed in the second and subsequent layers, a result of significant surface roughness in the underlying spacer layer. However by increasing the growth temperature to 580°C for the final 35nm of the 50-nm GaAs SPLs, a much smoother surface is achieved, allowing the fabrication of essentially identical, defect free QD layers. The suppression of defect formation enhances both the room-temperature photoluminescence efficiency and the performance of 1.3-μm multilayer InAs∕GaAs QD lasers. An extremely low continue-wave room-temperature threshold current density of 39A∕cm2 is...

The structural and optical properties of GaAs-based 1.3 μm InAs/InGaAs dots-in-a-well (DWELL) structures have been optimized in terms of different InGaAs and GaAs growth rates, the amount of InAs deposited, and In composition of the InGaAs quantum well (QW). An improvement in the optical efficiency is obtained by increasing the growth rate of the InGaAs and GaAs layers. A transition from small quantum dots (QDs), with a high density (∼5.3×1010 cm−2) and broad size distribution, to larger quantum dots with a low dot density (∼3.6×1010 cm−2) and narrow size distribution, occurs as the InAs coverage is increased from 2.6 to 2.9 monolayers. The room-temperature optical properties also improve with increased InAs coverage. A strong dependence of the QD density and the QD emission wavelength on the In composition of InGaAs well has been observed. By investigating the dependence of the dot density and the high-to-width ratio of InAs islands on the matrix of InGaAs strained buffer layer (SBL), we show that the increasing additional material from wetting layer and InGaAs layer into dots and the decreasing repulsive strain field between neighboring islands within substrate are responsible for improving QD density with increasing In composition in InGaAs SBL. The optical efficiency is sharply degraded when the InGaAs QW In composition is increased from 0.15 to 0.2. These results suggest that the optimum QW composition for 1.3 μm applications is ∼15%. Our optimum structure exhibits a room temperature emission of 1.32 μm with a linewidth of 27 meV.

The size and shape evolution of self-assembled InAs quantum dots (QDs) influenced by 2.0 ML InAs seed layer has been systematically investigated for 2.0, 2.5, and 2.9 ML deposition on GaAs(100) substrate. Based on comparisons with the formation of large incoherent InAs islands on single-layer samples at late growth stage, the larger coherent InAs quantum dots at 2.9 ML deposition has been observed on the second InAs layer. A simple model analysis accounting for the surface strain distribution influenced by buried islands gives a stronger increment of critical QD diameter for dislocation nucleation on the second layer in comparison with the single-layer samples. Additionally, the inhibition of dislocation nucleation in InGaAs/GaAs large islands can also be explained by our theoretical results. (C) 2000 American Institute of Physics. [S0021-8979(00)08922-2].

Optical and structural investigations of InAs quantum dots (QDs) covered by InxGa1-xAs (0 less than or equal to x less than or equal to 0.3) overgrowth layer have been systematically reported. The decrease of strain in the growth direction of InAs quantum dots covered by InGaAs layer instead of GaAs is demonstrated by transmission electron microscopy experiments. In addition, the atomic force microscopy measurement shows that the surface of InAs islands with 3-nm-thick In0.2Ga0.8As becomes flatter. However, the InGaAs islands nucleate on the top of quantum dots during the process of InAs islands covered with In0.3Ga0.7As. The significant redshift of the photoluminescence peak energy and reduction of photoluminescence linewidth of InAs quantum dots covered by InGaAs are observed. The energy gap change of InAs QDs covered by InGaAs could be explained in terms of reducing strain, suppressing compositional mixing, and increasing island height. (C) 2000 American Institute of Physics. [S0021-8979(00)04018-4].

The use of high-growth-temperature GaAs spacer layers is demonstrated to significantly enhance the performance of multilayer InAs/GaAs quantum-dot solar cells. Threading dislocations are observed for a 30-layer quantum-dot structure with GaAs spacer layers grown at low temperature (510°C). The formation of threading dislocations is suppressed by growing the GaAs spacer layer at high temperature (580°C), leading to enhanced quantum-dot optical and structural characteristics. Incorporation of the high-growth-temperature GaAs spacer layers into a 30-layer InAs/GaAs quantum-dot solar cell results in a dramatic increase in the short-circuit current compared to the one without the high-growth-temperature spacer layers and an increase in the short-circuit current compared to the reference GaAs solar cell. © 2012 American Institute of Physics.

The effect of growth temperature on the optical properties of self-assembled In0.65Al0.35As/Al0.35Ga0.65As quantum dots is studied using photoluminescence and electroluminescence spectra. With the growth temperature increasing from 530 to 560 degreesC, the improvement of optical and structural quality has been observed. Furthermore, edge-emitting laser diodes with three stacked InAlAs quantum dot layers grown at different temperature are processed, respectively. For samples with quantum dots grown at 560 degreesC, the continuous wave operation is obtained up to 220 K, which is much higher than that of ones with InAlAs islands grown at 530 degreesC and that of the short-wavelength quantum-dot laser previously reported. (C) 2001 American Institute of Physics.

We investigate the electronic energy levels and carrier dynamics in InAs/InxGa1−xAs dots-in-a-well (DWELL) structure by comprehensive spectroscopic characterization over a temperature range from 10 to 300 K. The integrated photoluminescence (PL) intensity is observed to increase with raising temperature up to 100 K. Through combining the PL and PL excitation (PLE) analysis, we provide direct evidence that this anomalous temperature dependence of the PL spectrum is due to the carrier trapping in InGaAs quantum well at low temperature. A rate equation model is adopted to quantitatively describe the thermal escape and capture processes of carriers in the DWELL system. The origin of thermal activation energies for quantum dot PL quenching at higher temperatures is discussed referring to the electronic energy levels revealed by PLE spectra.

We report a magneto-optical study of InAs-InGaAs-GaAs self-assembled quantum dots (QDs) subjected to post-growth thermal annealing at different temperatures. At low temperatures annealing strongly affects the bimodal distribution of QDs; at higher temperatures a strong blueshift of the emission occurs. Magnetophotoluminescence reveals that the annealing increases the QD size, with a larger effect occurring along the growth axis, and decreases the carrier effective masses. The main contribution to the blueshift is deduced to be an increase in the average Ga composition of the QDs. The inadvertent annealing which occurs during the growth of the upper AlGaAs cladding layer in laser structures is also studied.

A broadband midinfrared laser operating up to 340K in pulsed mode is presented. It was realized by incorporating seven different quantum cascade active regions with 0.2μm increments to emit at different wavelengths. Despite the relatively large wavelength increments, a continuous lasing spectrum is obtained from 7.7to8.4μm, with significant improvements in threshold current and temperature performance compared with previously reported broadband quantum cascade lasers. A full temperature dependent characterization was performed, and the threshold current density at 300K was 6.5kA∕cm2, with peak output power of over 100mW. A characteristic temperature of T0=192K was obtained.

In this paper we discuss a technique for broadening the emission and gain spectra of 1.3μm quantum dot superluminescent light emitting diodes (SLEDs). By incorporating different amounts of indium in different wells of a multi-dot-in-well stack we are able to tailor the emission and gain spectra of the devices. This technique allows us to overlap the ground state of one dot-in-well (DWELL) with the excited state of another to achieve broader and flatter emission spectra compared to a SLED design comprising DWELL layers of constant indium composition. Due to the low internal loss of these structures, this broadening is achieved without a significant reduction in the output power of the devices.

It is demonstrated that the emission of InAs quantum dots (QDs) capped with GaAsSb can be extended from 1.28 to 1.6 µm by increasing the Sb composition of the capping layer from 14% to 26%. Photoluminescence excitation spectroscopy is applied to investigate the nature of this large redshift. The dominant mechanism is shown to be the formation of a type-II transition between an electron state in the InAs QDs and a hole state in the GaAsSb capping layer. The prospects for using these structures to fabricate 1.55 µm injection lasers are discussed., This work is supported by the U.K. Engineering and Physical Sciences Research Council (EPSRC) (Grant No.GR/S49308/01) and the European Union Network of Excellence “SANDiE” (Contract No. 500101).

The optical and structural properties of InAs quantum dots (QDs) with a thin InAlAs–InGaAs composite cap layer have been systematically investigated by photoluminescence and transmission electron microscopy (TEM). A number of improvements in the optical properties are observed with the use of an InAlAs–InGaAs cap layer, instead of InGaAs. These include a redshift of the emission, a reduction of the photoluminescence linewidth, an increased separation between the ground- and first-excited-state transitions, and an enhancement of the photoluminescence intensity at room temperature. To understand these optical improvements, the structural characteristics of the dots are studied by cross-sectional TEM. The height of the QDs is found to increase with increasing InAlAs thickness in the InAlAs–InGaAs cap layer. In addition, scanning TEM is used to qualitatively map the Al distribution in the vicinity of the QDs. These studies indicate that Al atoms are not deposited directly above the QDs in the present structur...

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.

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 e...