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The effects of rapid thermal annealing on the optical properties of InGaAs-capped/uncapped quantum dots (QDs) with different areal density were investigated by photoluminescence (PL) measurements. Improved PL intensities were observed at optimum annealing conditions. However, the PL peak energy of the samples blue-shifts after annealing. The peak energy blue-shift strongly depends on QD areal density and capping layers. As compared to high-density QDs, low-density QDs are very sensitive to the annealing. At given annealing conditions (>700 0C, 30 s), peak energy blue-shift of low-density QDs is much larger than that of high density QDs. With increasing annealing temperature and/or time, PL intensity of low-density QDs significantly decreases. Capping QDs with an InGaAs layer, peak energy blue-shift of the samples is further increased. We attribute the increased PL peak energy blue-shift to enhanced strain-driven diffusion and/or defect-assisted diffusion.
Due to unique three dimensional quantum confinement and their potential device applications, semiconductor quantum dots (QDs) have attracted considerable interests in the last three decades. Using the Stranski-Krastanow growth method, low threshold, high thermal stability, and high quantum efficiency lasers, and other active components have been demonstrated in In(Ga)As/GaAs QD systems.1-4 Rapid thermal annealing (RTA) plays an important role in semiconductor QDs, because of the large surface area to volume ratio compared to the bulk or quantum well structures. As the dimension of QDs is typically about a few tenth nanometers, a small interdiffusion between the dots and the surrounding materials is expected to produce a significant change in the band structure and the optical properties of the QD material.5 Recently, the capability of modifying the optical properties of QDs by RTA has been applied with promising results, including controlling fine structure-splitting of individual InAs QDs and improving device performance of QD infrared photodetectors.6, 7 As an efficient approach of modifying the electronic states of the QD assemblies, thermal annealing of QD systems has become a subject of intense investigation.8-15 After RTA, photoluminescence (PL) peak energy blue-shift as well as line-width decrease is typically observed. In order to reach technical important telecommunication wavelength of 1.3 or 1.55 µm, QDs are generally capped by an InGaAs layer.16 Although RTA of such capped QDs has been done in several reports, there is lack of comparative studies of RTA effects on capped and uncapped QDs.8 Furthermore, so far RTA of QDs is mainly investigated on high-density QDs due to their potential applications in real devices.4-13 There are few reports about RTA of low-density QDs.14, 15 It was recently demonstrated that low-density QDs is a very promising candidate for realizing telecommunication wavelength single photon emitters. 17 Thus, RTA of low-density QDs becomes increasingly important.
In this letter, we reported the effects of RTA on the optical properties of InGaAs-capped/uncapped QDs with different areal density. Room temperature PL measurements were used to evaluate the effects. Upon RTA, PL peak energy blue-shift strongly depends on QD areal density and capping layers. The underlying mechanisms which are responsible for the PL peak energy blue-shift are discussed.
The samples used in this study were grown by molecular beam epitaxy on semi-insulating (100) GaAs substrates. Low- and high-density QDs with areal density of about 4-108 and 1.7-1010 cm-2 were prepared by the method described in Ref. 17. The growth rates of low- and high-density QDs are 0.002 and 0.1 monolayer/s, respectively. In this study, samples with different types of structures were used. There are five samples, i.e. a low-density and a high-density QDs embedded in a GaAs matrix, a low-density and a high-density InGaAs-capped QDs embedded in a GaAs matrix, and an InGaAs /GaAs quantum well (QW). In order to make comparisons, identical layer thickness of 5 nm and In composition of 15% are used for the InGaAs QW and InGaAs capping layer.
RTA was carried out on the samples sandwiched by two protective GaAs substrates in a RTA system that uses halogen lamps and a flowing nitrogen gas ambient. The temperature was measured by a thermocouple attached to a Si sample holder. The annealing temperature and time were varied from 550 to 900 °C and 30 s to 30 min, respectively. The RTA for different temperatures at a fixed annealing period and the RTA for consecutive time intervals at a fixed annealing temperature were carried out on different cleaved pieces of the same sample to avoid scatter due to possible material variations. After annealing, the samples were characterized by PL measurements. The PL was excited with a 632.8 nm He-Ne laser and monitored by an uncooled InGaAs detector at room temperature.
Results and Discussion
Typical PL spectra of the annealed samples taken at annealing temperature of 750°C for 30s are shown in Fig.1. Spectra of the as-grown samples are also shown as references. All spectra are normalized for clarity. As compared to the as-grown samples, significant PL peak energy blue-shifts from the annealed QD samples were observed, which is different from the annealed QW samples. For the QW sample, the PL peak energy blue-shift is nearly negligible. The PL peak energy shift after RTA are generally attributed to the In/Ga atomic inter-diffusion across the interfaces of the QDs/QW and their surrounding matrix.4-13 The interdiffusion can be influenced by several factors, such as annealing temperature, annealing time, dielectric encapsulation,8-10 strain-driven diffusion,18 defect-assisted diffusion8-10 and so on. In this study, strain-driven diffusion is believed to be mainly responsible for the observed significant PL peak energy blue-shift due to a highly accumulated strain in the QD structure.
Figure 2 summarizes the PL peak energy shifts from all the five types of samples annealed at different temperatures for 30 s. The PL peak energies of all the samples exhibit nearly no shift after annealing at the temperature below 650 °C. However, distinct peak energy shifts are observed after annealing at the temperature above 700 °C except for the QW sample. The peak energy shift of the low-density QDs, either with or without InGaAs capping layer, is larger than that of the high-density QDs. Upon annealing at 850 °C, the energy shift of low-density uncapped QDs is as large as 250 meV, whereas the peak energy shift of high-density uncapped QDs is only 140 meV. The significant energy shift difference probably originates from enhanced strain-driven diffusion due to the larger QD size and higher In composition of the low density QDs. In the presence of the InGaAs capping layer, the peak energy shifts of the capped QDs are further increased. Upon annealing at 900 0C, energy shifts up to 410 and 260 meV were respectively achieved from low- and high-density InGaAs-capped QDs. It was argued that InGaAs capping layer relaxes the strain in the QDs due to reduced lattice-mismatch between QDs and (In)GaAs capping layer,16 opposite trend might be expected. However, an increased In composition in the InGaAs-capped QDs has been reported due to suppressed In segregation.19 Although the strain of the QDs is partly relaxed by introducing the InGaAs capping layer, the composite strain of whole the QDs and InGaAs capping layer system actually increases. The increased strain might be able to enhance In/Ga atomic interdiffusion due to strain-driven diffusion, and consequently leads to an increased PL peak energy shift.
Integrated PL intensity ratios of annealed and as-grown samples as a function of annealing temperature are depicted in Fig. 3. As generally reported, the PL intensity ratios initially increase and then decrease with increasing annealing temperature; the optimum annealing temperature is about 650 °C which is nearly independent on QD density and sample structures. The increase of the PL intensity ratios is attributed to the removal of defects and impurities in the QDs/QW and heterointerface regions, while the decrease of PL intensity ratios probably result from weak carrier confinement and/or annealing-induced defects in the higher temperature annealed samples.13 At optimum annealing temperature, PL intensity ratio of the QW sample is remarkably improved by a factor of about 7 with a nearly negligible PL peak energy shift as shown in Fig. 2. In the contrast, the PL intensity ratios of the uncapped QDs are only improved by a maximum factor of about 2 and then drastically degraded as the annealing temperature increases. The observations might imply an inferior material quality of the InGaAs QW. There must be an existence of nonradiative defects in the InGaAs layer due to unoptimized growth conditions.20 Fortunately, they can be removed by RTA.
In order to investigate the effects of annealing time, RTA was done at annealing temperature of 650 0C for consecutive time intervals (from 30 to 1800 s). Fig. 4 shows PL peak energy shift of the QD samples as function of annealing time. For the uncapped QDs, PL peak energy shift is very small (<5 meV?) in the range of investigated annealing period. However, for the InGaAs-capped QDs, the PL peak energy shifts change rapidly in the initial stage of RTA and then gradually increase with annealing time. Significant PL peak energy shifts up to 50 and 120 meV were respectively observed from the high- and low-density InGaAs-capped QDs annealed for 240 s. It was demonstrated that, in the presence of defects, atomic interdifusion in a semiconductor nanostructure could be enhanced by defect-assisted diffusion.8-10 It is likely that the diffusion rate of In and Ga atoms is very high at the beginning of RTA for the defect-rich composite QD structure due to the presence of InGaAs-capping layer, but gradually decreases as the defects are removed by annealing.
The evolution of the QD optical properties as a function of annealing time are shown in Fig. 5. For clarification, PL spectra of the samples were normalized and only part of the spectra from the low- and high-density InGaAs-capped QDs is presented. As the annealing time increases, PL intensity of the high-density QDs first increases and then decreases, while PL intensity of the low-density QDs monotonically decreases. After annealing for 720 s, PL intensity of the low-density QDs decreases by a factor of about 10. This is contrast to that of the high-density QDs for which the PL intensity increases by a factor of about 4. The observations may indicate that low-density QDs are very sensitive to thermal annealing, which has been confirmed in the fabrication of single photon emitters. In order to preserve a high material quality, it was suggested that, in a real device structure, the layers on top of the low-density QDs are better grown with a temperature not higher than that for the QD growth. Except for the PL intensity variation, PL spectrum shape of the high-density QDs also evidently changes. Ground-state emission predominate the PL spectra of the low-density QDs. However, after annealing, a higher energy emission envelop becomes dominating in the PL spectra of the high-density QDs as the annealing time increases. The observation is very similar to that reported by Kobayashi et al..10 We attribute the appearance of the higher energy envelop to QD size dispersion, which could be understood by considering the following scenario: In/Ga atomic interdiffusion might be nonuniform for individual QD in the QD assemble. Due to the nonuniformity, some of the QDs may dissolve faster than the others. As a consequence, a local QD size variation might be induced due to a higher QD density.
The effects of RTA on the optical properties of InGaAs-capped/uncapped quantum dots (QDs) with different areal density were investigated by PL measurements. After annealing, an improved PL intensity and a blue-shifted PL peak energy were observed. The PL peak energy blue-shift strongly depends on QD areal density and capping layers. At given annealing conditions (>700 0C, 30 s), the peak energy blue-shift of low density QDs is much larger than that of high density QDs, probably caused by enhanced strain-driven diffusion due to the larger QD size and higher In composition of low density QDs. With an InGaAs capping layer on the top of the QDs, the energy blue-shift is further increased due to defect-assisted diffusion. As compared to high-density QDs, low-density QDs are very sensitive to the RTA. The PL intensity of low-density QDs drastically decreases as annealing temperature and/or annealing time increases. The results are not desired, but they provide very useful information for improving performances of real devices which is based on low-density QDs such as single photon emitter.
The authors would like to thank Dr. Mark Stringer (Uni. of Leeds) for his help in setting up the PL system, and Dr. Li Chen (Uni. of Leeds) for technical supports in using the RTA system. The works were partly supported by xxx grant.
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